Why would humans be evil if they become lower plane petitioners?

It definitely escalated faster than they expected, but we can't say if that's a statement on human nature or poor experiment design because there were simply so many flaws.

Oh yeah, definitely. There are very few people that would ever consider themselves evil, most assuredly. I don't know how much I'd agree with the "lawful" part, though; personally, I'd say most people are just neutral overall. But mostly yeah I'd agree with you.

When you're talking about laws that aren't explicitly associated with morality like that, I can agree with that. And on the other hand, for laws more obviously based on a moral code (like "don't murder" or "don't steal"), I'd say that obeying those laws says less about your position on the L/C axis and more about your position on the G/E one.

With my DM hat on, I would generally characterize theft in D&D as nonlawful, rather than nongood. Obviously this depends on the motivations of the character.

I would have no problem with the concept of a Chaotic Neutral character who steals often. (I would also characterize some NPC bandits/pirates as being Chaotic Neutral.)

See, this sort of discussion is the thing I love about D&D's alignment system. There's so much room for personalization, for interpretation, for understanding; naturally, this also leaves room for misunderstanding and arguments when players disagree with each other...

Idran, everyone knows the Teletubbies are Chaotic Evil. Get with the program.

The Stanley Milgram Experiment is an excellent example of how a person can be persuaded to do something awful, even though they know it's wrong. IRRC, at the end of the experiment they interviewed the subjects; they assured them that it wasn't real, that no one was hurt, and asked them questions about their experience. Most were visibly shaken. Most seemed to indicate that they were appalled by the behavior of their "superiors." But most also went through with the experiment; only two or three actually walked away from it.
Yes, but in D&D terms, those people felt guilty and would have sought atonement, thus no alignment change, though they might have permanently lost class benefits. In the D&D alignment system, when someone's alignment changes from multiple evil acts, this essentially represents desensitization or addiction to the thrill/etc. For some people, all it takes is one time and they start sliding irresistably towards evil (or at best, neutrality).

With my DM hat on, I would generally characterize theft in D&D as nonlawful, rather than nongood. Obviously this depends on the motivations of the character.
That's the rule, yes. There are plenty of chaotic good thieves and ruffians-- Robin Hood, Lupin the Gentleman Thief, the Three Muskateers, and Zorro being examples, along with quite a few outlaw cowboy characters. CG thieves tend to choose only other criminals, pirates, and corrupt government institutions as targets of theft. Robin Hood in particular targetted swindling merchants and corrupt officials who robbed what was rightfully (in Robin Hood's mind) the property of the poor-- Robin Hood was merely righting a societal wrong in his own way. Lupin IIRC did this as well.

For that matter, can you be a good prince if your kingdom is supported by slavery? Even if you are nice to your slaves, aren't you at minimum tacitly controlling the slave trade, and if you take the throne are you now actively supporting it by not freeing all slaves?
That depends on what kind of government the prince runs and whether or not the King is alive and the actual ruler. In more Republican monarchies, there's a possibility that the prince might have to wrestle with state officials and the church to outlaw slavery.

Just so long as we avoid "what alignment is (this fictional non-D&D character)" as a topic; that seems to lead to the worst shouting matches.
Some characters, like Batman, are difficult to pin down, while others, such as the types I mentioned above, are quite clearly CG.

I gather from this dicussion (and many others on various forums, all about alignment and how it is/isn't a good system) that for some characters, it's possible and easy to play according to the alignment; while for others (for most, I find) it's not, and alignment should only be one of many guideposts for informing character decisions.

In the case of the OP, most villains don't choose to be evil: they just are. They have their own motivations, they think of themselves as good or neutral, and they typically champion a cause of some kind, whether personal, for others, or for many. I saw a horror movie last night where the villain was part of a cult that developed a weaponized virus; his motivation for wiping out most of the human race was to protect the planet. His motivation alone suggests a chaotic neutral or even chaotic good alignment, but he was willing to kill literal innocents (normal people who had no direct hand in destroying the planet) to accomplish his goals, which suggests an evil component. Good, evil, neutral, lawful or chaotic? I don't know; as a DM, I'd have to let a character like that play for several sessions before I start questioning whether his alignment (as written) is accurate.

When he dies, though, a decision needs to be made, in the context of Planescape, to decide where he goes.

Another thought: what happens between death and assignment to an outer plane? Is there a sort of judging? Does a person go through additional trials to determine their true alignment? What people are truly good, but they've never lived a life where they could test their moral resolve? Are they considered neutral because of their actions, or because of the strength of their hearts? Is there a trial by a power of justice? Or is there a council made of different representatives, in order to determine a fair planar-petitioner assignment?

^^That's how the animentals were written up in the Planescape Monstrous Compendium III.

I saw a horror movie last night where the villain was part of a cult that developed a weaponized virus; his motivation for wiping out most of the human race was to protect the planet. His motivation alone suggests a chaotic neutral or even chaotic good alignment, but he was willing to kill literal innocents (normal people who had no direct hand in destroying the planet) to accomplish his goals, which suggests an evil component. Good, evil, neutral, lawful or chaotic? I don't know; as a DM, I'd have to let a character like that play for several sessions before I start questioning whether his alignment (as written) is accurate.
Actually, that villain would be evil, and fits the general motivations of the typical NE druid.

Another thought: what happens between death and assignment to an outer plane? Is there a sort of judging?
The planes themselves do the judging. The soul goes though the Astral and then is drawn like a magnet (not through any power or will of the soul itself) to an Outer Plane. If they are the worshippers of a particular deity, then they are drawn to said power's divine realm. The whole process is normally automatic, though petitioners on a divine realm may go through some sort of judgement.
With cultists it depends on the degree of lip service they gave, but the general rule is that at the very least, they'll end up as petitioners on that quasipower's layer or plane. Most quasipowers do not automatically gain petitioners; for such quasipowers, the only way to acquire petitioners is to get them to make special pacts (generally, promising their soul to the quasipower) Evil quasipowers make up for the lack of petitioners by consuming the souls of sacrifices, but presumably this is not nearly as efficient as absorbing actual petitioners, since sacrifices don't usually get the chance to 'ripen' like petitioners do-- thus most of what makes up that sacrificed soul is mere chaff. To give you an example, a demigod typically has between 500-1000 worshippers on a single given prime world at any one time, while a quasipower typically has between 50-250. Those few cultists of evil quasipowers, or at least the higher-ranking members, will each sacrifice hundreds to thousands of victims within their lifetime-- especially true of the priests and templars of the cult. So you have to figure that among those 50-250, a good 10% of them are members of the priesthood/cult leaders. At minimum, a single priest is likely to sacrifice one or two sentient victims per year, and you have to figure that the average priest will be in service for 10-20 years. That means that a single cult sacrifices... 100 to 500 victims in a lifetime, and you have to assume that in the same 20 year period, that about a third of a demipower's worshippers pass on, which equals 155~333 petitioners during the same time period.

When you refer to a quasipower, are you referring to powerful exemplar beings such as Orcus, Graz'zt, Demogorgon, etc?
Correct. Alongside Talisid and his companions, Primus, the Slaad lords, the queen of the Eladrin/Court of Stars, the Archomentals, the children of Tiamat and Bahamut, etc.

Also, what does the canon have to say, if anything, about what happens to the soul of someone who is sacrificed to such a being?
It depends, I thought there was something in Kostchtchie's Demonomicon entry that said that sacrifices who accept mercy get devoured immediately by the prince and thus no afterlife, but I seem to be mistaken. Here are some excerpts:

From Dragon 357:
"Demogorgon vastly prefers the souls of his enemies-- good-aligned clerics, paladins, and rangers who devote their lives yto the destruction of all things demonic are widely known to be his favorite, and many of his cultists go to unusual lengths or great risk to capture such sacrifices.
...
With a properly observed ritual, the soul of a person sacrificed in this manner is carried directly to Lemoriax, where it is steeped in the vile Fleshforges and transformed into a larvae, even if the soul itself was not destined for such a fate. This is one of the few instances where an innocent soul can become a larvae.

From Dragon 341:
"To a minotaur cultist, the murder of gnolls is the highest form of respect it can offer the Prince of Beasts. They believe that each gnoll slain is one less gnoll in the mortal world, and one more gnoll in the Abyss for Baphomet to personally destroy"

From Dragon 345:
"Once the appropriate amount of time has passed, the cult leader approaches the woman and gives her an opportunity to accept Kostchtchie into her heart. Those who do are immediately executed with a large, heavy bludgeoning weapon, and their spirits are shunted directly into the Iron Wastes, where they become lodged in fields of glaciers and ice. These spirits are left to languish in eternal torment-- when the Prince of Wrath does visit them, it is only to mock and torment them.
....
When the sacrifice refuses mercy, the high priest summons the monster (usually in a mundane manner, such as a drum or gong). The creature arrives within minutes, tears the sacrifice loose, and returns with the woman to its lair to eat or torment her. Of course, once the sacrifice is slain, her soul is free and is not damned to the Abyss-- the reward for these souls is passage into the afterlife.
The loss of these souls annoys Kostchtchie, but he takes solace in the knowledge that their ends were particularly painful and gruesome."

From Dragon 333:
"Eventually, the victim realizes the truth-- by which point it is too late. Only then does Frazz-Urb'luu pay the victim's soul a personal visit to explain just what he has done. The Prince uses his powers of illusion to grant a vision of how the sacrifice's delusions have harmed those he loved in lifde and allowed evil and chaos to spread, and finally, when the victim has sunk as far as he can into despair, he is consumed by the demon lord. Characters slain in this manner cannot be brought back without direct divine intervention."

From the Fiendish Codex 1:
"Demon lords who control a layer of the Abyss have a special relationship with their mortal followers. When chaotic evil worshipers of these demon lords die, their souls manifest on the home layer of their demon lord, usually as manes.
The souls of those foolish enough to have entered a demonic pact that promised their soul to the demon lord suffer the same fate.
Some demon lords sacrifice a portion of their dedicated souls to maintain a hold on their Abyssal layer, whereas others press them into service in huge armies that defend their holdings from the predations of rival demon lords."

From the Book of Vile Darkness:
"In some abstract sense, the sacrifice adds some small mote to the god’s overwhelming power, and in return the evil deity is sometimes willing to grant a boon or a blessing in return.
...
When that blood spills during the ritual murder of an innocent creature, the blood (and sometimes even the soul of the creature) is dedicated to a nether power of malevolence.
.....
Occasionally, Orcus allows his wand to be found by a mortal in order to wreak greater chaos and evil among those inhabiting the Material Plane. This sort of dalliance lasts only a year or two at most—before the bloated prince grows bored and reclaims his artifact, usually along with the soul of whoever currently wields it."

From Fiendish Codex II:
"The Styx forever erases the memories of any soul coming into contact with it, and (newly arrived petitioners) yield much more divine energy if their identities are intact when their torment begins.
....
the soul shells undergo an awful program of torture, the gruesome details of which are best left to the imagination.
While slowly peeling away every last iota of the petitioner’s individuality, the process releases magical energy, which flows to the local lord as specified in the Pact Primeval."