[Wild Goose]

Regarding guilt levels etc, as this is a free-fire zone:

Spoiler for Ultimate fate of PKers:

It's generally a moot point, as most PKers tend to get killed in self defense. Players who've killed ingame are counseled and undergo psychiatric evaluation - those like Kirito, who kill in self defense, are let off. Those PKers like Laughing Coffin survivors are kept for a year of observation, part mental hospital, part juvie-ish detention area.

As for the killing itself, at least under British Law, which I studied, you'd need to prove a certain chain of causation, and pull in the Reasonable Man test for loads of things. Is Kayaba's assertation that death ingame is reasonable and believable? - among other things.

As for mens rea, the barest minimum you need is to prove the intent to cause grievous bodily harm, and the actus rea is the act itself. Mens rea may not be provable, but it should be possible to establish causation by using server logs - time D PKs V is recorded and shows cause of death, and system establishes D is the killer, then in the real world the NervGear kills the player.

But since mens rea may not be provable, you'd get Manslaughter instead. (i'm simplifying a lot from what I studied.)