When life becomes a monotonous cycle, it's not hard to grasp how someone could be pushed to the brink of violence, all in the name of feeling free. It becomes even more understandable if that person never even began to live, before being shoved out of the nest. With no gentle way out, the only chance to start anew is to snap chains and splatter organs.
Putting it that way, it was only a matter of time until Demian gave in. We can't prevent a tragedy, but we can cause their descent.