Complex sentence: No one, he (in what's left of his damaged mind) knows, which is why he suffers a complete psychotic break, and after venting his fury on a nurse, turns his rage towards those who left him this way.
Split into simpler sentences: No one, he (in what's left of his damaged mind) knows that. Which is why he suffers a complete psychotic break, and after venting his fury on a nurse, turns his rage towards those who left him this way.