I don’t think drugs, guns, TV, games, or any other inanimate items cause this sort of behavior. I think these people were broken inside to begin with and they freely chose to do sick, bad things. The drugs were used because of the way these people were to begin with; I do not think drugs made them what they are or caused them to do what they do. It would be a pretty big leap to decide to kill people if you aren’t inclined to do that to begin with regardless of what kind of substance you’re taking.
Why do some people choose to kill simply for the sake of killing? That’s a question asked since forever ago and the material world cannot explain it. Material objects like guns and pills do not “do” anything. Just my 2 cents for whatever its worth.