Even if he wasn't in his right mind, and the psychosis is entirely to blame. It's still his fault.
I've stayed up for 6 days on plain old amphetamine before, yeah I went batshit crazy and did some stuff that makes absolutely no sense now. (lets turn all the electronics on in the house so people can't listen to me..)
But, it didn't make me go out and steal/kill a goat, I never got into trouble with the police. I just lost my mind a bit and kept to myself. If he can't handle his shit he shouldn't be doing fucking stimulants - blame them all you want. But he took them, he carried out these grim acts. And it's his fault.