" A smorgasbord of questions, concerns, theories and frustrations
It appears that Ammit goes by a very different idea of what "balanced" and "unbalanced" mean compared to Taweret and the powers that be governing the afterlife. Ammit's concept of "unbalanced" seems to be someone who inflicted harm in any way, for any reason, under any circumstances. If that were really the case, I find it hard to believe ANYONE would ever pass the test and survive judgement, so she must make exceptions somewhere. The only exception she canonically makes is obviously Harrow, because despite being unbalanced she sees that he can be useful to her. So it would follow that Ammit overlooks unbalanced souls who she sees will be helpful* to her in the future, thus ensuring all of humanity isn't wiped out because, well, nobody's perfect. Khonshu's line to Ammit about Harrow, "Loyalty at what cost? An empty world for your disciples to inherit?" lends credence to this theory.
However, the fact that Harrow's scales are unbalanced because of his future service to Ammit implies that Ammit may not be fully in control of the scales after all. It seems that if she had it her way, a "balanced" soul would be someone loyal to her, even if they did evil things in her name, and "unbalanced" would mean someone disloyal to Ammit. For example, Alexander the Great's scales supposedly balanced perfectly, but in the end he "betrayed" Ammit, which lowered him in her estimation.
It's also possible that the criteria of the of the scales varies according to the ethical and behavioral norms of the person's culture. I don't think this is likely, but I thought it worth mentioning as a theory.
*"Helpful" =/= Loyal or devoted, because if that were the case none of the Ammit followers at Harrow's rallies would have been killed. For instance, that old woman in Episode 1 had "been good her entire life" and was clearly a devout worshipper of Ammit, but she was still judged as unbalanced. Unfortunately, this may have been because she was getting older and frankly wouldn't have many decades left to serve Ammit when she was awakened, or the physical ability to do so.
In trying to decode Ammit's definition of "sin," we're at a disadvantage in that we don't know the backstories (or "futurestories," I guess) of most of the people she judged "unbalanced." Exactly what kinds of sins did they commit (or were destined to commit in the future)? It doesn't help that whenever Harrow talks about this, he does so in a really vague way. He doesn't say, for instance, "Ammit plans to eliminate everyone destined to commit first-degree murder." Instead, he says "Ammit will light the path to good by eradicating the choice of evil," which is very sexy of him, but not very informative. (Sidebar: I feel I should add that this ambiguity does serve a narrative purpose; if the audience isn't told exactly what Ammit thinks is evil and what kind of "sinners" she's punishing, it's easier to see Harrow as morally grey rather than a complete villain. As a fan of good character writing, I love it, but as a bratty lore-scientist I'm frustrated).
All that being said, there is one exception to this "vagueness" Harrow adopts when speaking about Ammit's judgement, and it's in Episode 1 when he corners Steven in the museum and tells him Ammit would have prevented the crimes of Hitler, Nero and Pol Pot, as well as the Armenian genocide. This is, as far as I can tell, the closest we get to a specific explanation of what Ammit considers sinful. So we know she doesn't like those who abuse their power to commit wide-scale mass murder, though most people would probably agree that this should go without saying anyway. The other problem is we have to remember Harrow had an agenda with Steven in this scene, and probably chose to give those four examples precisely because they were so prolific and widely recognized as terrible things done by evil people - basically, appealing to pathos. Would Ammit really have killed Hitler and prevented the Holocaust? Probably, but that's not really Harrow's point. My best guess is that Ammit does kill genocidal dictators, but for every genocidal dictator she also kills a million more people who committed nonviolent crimes, or violent crimes with extenuating circumstances like self-defense, or those who would have sincerely repented and changed their ways years after their crime. So Harrow was doing what Harrow does best, which is technically telling the truth, but only as much of the truth as will get him what he wants. Again, extremely sexy of him, but not very helpful.
TLDR, maybe Ammit really does care about creating a peaceful world. Maybe she really does believe the ends justify the means, "means" in this case being "not killing unbalanced people as long as they can help me somehow" and "ends" being "they eventually help me create the perfect world and then I kill them anyway, oops." Or maybe what she really cares about is being worshipped. Who knows.
So anyway, that's Ammit's scales. But then we get to Episode 5 and it turns out there are REAL scales in the underworld, and from what I can tell just from Marc and Steven's case alone, these scales go by different criteria. Marc and Steven's scales were unbalanced because they weren't being honest with each other and with themselves about crucial events in their lives. It didn't seem to come down to whether they were "good" or "evil" people. Even when Marc's actual crimes were brought up, their purpose wasn't to guilt him into atoning for his sins, but to force him to be honest with Steven. Marc's scales only balanced when Steven was "neutralized" (for lack of a better word), thus putting to rest the "chaos" inside him.
In what I've read about the traditional scales of judgement in Egyptian mythology, the most consistent explanation seems to be that an "unbalanced" soul is one that has more evil than good in it. So presumably, someone who had done evil things could make up for it by doing a ton more good things, and still end up with balanced scales. But that's definitely not how Ammit works, and Taweret really doesn't seem too concerned with good and evil anyway.
Moon Knight had an Egyptology consultant on set, so the showrunners almost definitely knew this stuff about the scales, as well as the fact that the actual god in charge of judging souls was Anubis, and that Ammit was a soul-eating demon who worked under him. In keeping with their goal of remaining respectful to the culture and mythology of Egypt even while taking the occasional creative liberty, they would have certainly taken these pieces of lore about the afterlife into consideration.
What I ultimately take away from all this is that the judgement system in the MCU Egyptian afterlife isn't working the way it was originally intended to, and it's having serious consequences for both the gods and humanity.
Okay, now for my own personal theory: Anubis was originally the arbiter of the underworld, and judged souls as balanced or unbalanced based on whether the good in their hearts outweighed the evil. If they were judged as evil, they were fed to Ammit, and eventually Ammit developed a taste for evil souls. She pulled strings, maybe did some blackmailing, and got the Ennead to imprison Anubis, leaving Ammit as his "understudy" in charge of judging souls. But she stopped using the old method of judgement, and crafted her own set of scales that would judge souls as unbalanced if they had done ANY harm at all during their lives. Things inevitably got out of hand, and the Ennead imprisoned Ammit as well, which is how Taweret got the gig.
Here's a possible premise for Season 2: Ammit is gone for good (or so everyone thinks...wink...), but still no one, god or human, is happy with the way the scales system works. Taweret is used as a scapegoat and imprisoned in an ushabti. Now it's complete anarchy down in the Duat, so the gods are forced to re-release Anubis. BOOM. You're welcome, Marvel.