JVDS is not criminally insane. He knows right from wrong. If fact, he ran to Chile after the murder, which is consciousness of guilt (i.e. knowing he committed a murder and knowing it was wrong). Further, he attempted to disguise his appearance by lightening his hair, another indicator.
We've never going to know what exactly set him off. Why? Because he's never going to tell the truth! It could be anything...or nothing...he is known to have a hair trigger temper. He has no impulse control. He is also known to be a pathological liar--and likes making a game out of lying and then lying some more.
So all of this is interesting, but ultimately a fruitless exercise in guessing with no confirmation.
But: he will not get away with the crime, he is not insane, he knows right from wrong, he does display what is considered psychopathic tendencies, he is remorseless, he is a liar, and he will not ever change at his core. He is what he is. He was that way when he was 13 and he's the same today (just larger and even more dangerous).
He will do just fine in that Castro Castro prison--he's physically larger than everyone else, just as cunning, will get more cunning and cutthroat in the days ahead, will make his little deals on the side, and will probably have a few guards eating out of his hands eventually. Don't you (all) worry about poor little JVDS. He is not a lamb to the slaughter--he earned his way into that prison and will fit in quite well.
Well said, thanks!