I agree that the man is clearly off his rocker, but from a legal perspective, what matters is whether or not you understand that you have, in fact, killed a dude, and that this is in fact a bad thing. Having voices in your head telling you to do it is a completely separate issue. Again, this is my understanding as a layman, so any actual lawyers please feel free to tell me I’m full of shit.
“No, John. You are the demons.”
And then John was a zombie.