So, I think it's important first off to distinguish moral/immoral from villain/not. A character can behave profoundly immorally without necessarily being a villain, and I believe a character can potentially be a villain while at least loosely following a moral course of action.
Moral/immoral has some points of debate but my general standpoint is:
1. trying to survive is in itself morally neutral
2. benefiting yourself at the unwilling expense of others is immoral
3. assisting others, especially at cost to yourself, tends to be morally good
This does not change because someone is pointing a gun at your head and telling you to be immoral or they'll shoot. That action of theirs is wildly immoral but the AT doesn't care. Maybe they even keep score of who's the baddest dude!
The problem in SOTF is that pursuing survival often leads characters to victimize others. This remains immoral. There are mitigating factors, and it can be plenty understandable and sympathetic (and most of the best characters do it some and real people do it in tiny ways constantly for lower stakes) but this does not make it moral.
We then segue into the rules of SOTF:
1. You have to be the last alive to win
2. You have to kill at least one person to go home
3. At least one person a day must kill or everyone dies
Even taken at face value, this does not justify murder in 99% of circumstances, and that's putting aside that the AT is almost certainly lying about rules 2 and 3. It's quite possible to coast through the game, score your kill in self-defense or as mercy, and go home by the rules laid out by the AT. This is more or less what a moral game looks like. It's pretty boring.
Villainy is when characters take on antagonistic roles. This can be small-scale (targeting one specific person) but in general to be a villain in SOTF you take an antagonistic stance towards a significant chunk of the class. It doesn't have to be everyone, but a villain is usually someone you may well think "Uh oh" about if they roll into your thread.
To cross the line into being a villain instead of just doing bad things, I think a character needs to on balance do villainous stuff a good proportion of their time. Having a villainous "edit" also helps. So if you take, say, Parker from V7, I think there's a good argument he's a villain (and immoral!) because he repeatedly tried to turn events to his advantage, worked every angle he could, and did so with no concern about how it'd hurt others. He didn't screw over other people because he enjoyed doing it (mostly
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Ace, by contrast, has done some wildly immoral things lately, like murdering Connor. He has an even wider number of, let us say, jerk moves (running off and leaving Sakurako). That said, the only point I'd lean on as him being villainous would be when he antagonized Myles and tried to force his way into the group. This is a rather petty evil, but it's a more calculated and personal one and was the work of more considered bad stuff. Also, when Ace turns up in a thread, it's rare that I expect him to cause trouble. He escalates situations but until recently at least for most of the class he wouldn't be too likely to mess with them unprovoked.
Where this falls with mid-range killers is hard to say, but I think you can be a villain and also relatable, sympathetic, human, and someone the reader cheers on. Does that need its own category? TVTropes used to have one I think, but I dunno that I need to draw a line. I have a whole other screed on vilalins but I think part of what makes allegations of villainy so contentious is people read that as shorthand for being irredeemable, unsympathetic, or purely vile... when in fact, to me at least it's no such thing. Furthermore, being labelled a villain is not a negative critique and doesn't restrain a writer's choices. It's not a bad thing. I think the site forgets that at times.
Quick off-the-cuff stuff here—more thoughts later maybe.