I think it's much too easy to draw conclusions that one thing causes another, just because those two things coexist. A murderous psychopath smokes pot? People jump to the conclusion that there is a causative factor there. It would be just as easy to say he smokes regular cigarettes, or drinks beer, or drinks water (something in the water supply), or his mother's middle name begins with an R. Studies have shown that there is a connection? Bingo, we've solved the problem!
To me, it makes much more sense to say mass shootings are caused by cruelty, child neglect, hateful, baiting politics, racial/religious prejudice, ignorance of warning signals, etc. To the best of my knowledge, there is absolutely no evidence linking weed and violence. If anything, it probably works the opposite way, by making heavy users too "relaxed" and laid back to do anything whatsoever!
I agree there are alot of contributing factors to the manufacture of a mass murderer. Probably takes a perfect storm of events, variables, mental health, family issues etc.
What I see with pot is I think there is a misnomer that it only makes people mellow which from personal experience I've found to be the opposite or not the only thing it does. Same for many of these psychiatric drugs like SSRIs etc. Sometimes pre existing conditions like personality, life events & experiences and personal health will offset any of the desired effects of these drugs. And which is why heavy/regular use shouldn't be ignored on any level.