Well, there is drug and drug. I suspect that some substances are more suspect than others. Almost each time there is an unexplained shooting, especially if the killer shot his loved ones, I find an instance of drug use. And I don't mean just the illegal ones - in fact the legal ones may be worse. Too bad that this part gets overlooked each time - where is the autopsy of the killer? Was he high that day? On what? Was he following some medical cure? What was given to him?

But you are right, there is an extra ingredient: the guy had an intent to kill. And here is where the drug enters in the scene: the most dangerous drug is the one that takes off the inhibition to kill.