I'm willing to listen to a case that a sane, reasoning, emotionally mature, non chemically-imbalanced individual would logically conclude that a school mass shooting is a good idea, but I don't see one myself.
However, I certainly agree that doesn't imply all mentally ill people are violent.
Um. "I can't think of another reason, therefore this is the cause" is kinda shoddy reasoning. But since you asked: Factors far more common than mental illness include substance abuse and being the victim of bullying
has some very interesting notes on mental illness and violence: Among other things, it notes that rates of violence among the mentally ill are indistinguishable
from the general population - in fact, "young, poor boy, therefore violent" (while also a terrible methodology) would have a significantly higher hit rate. It's pretty obvious that "Mass murderer was a poor, aggrieved white dude with a substance abuse problem who recently had a major life setback" is way
more accurate than "mass murderer must've been mentally ill." It also notes that violence is negatively
correlated with schizophrenia and delusion - some of the most stigmatized mental illnesses - as compared to the general population.
And while we're at it, violent victimization is a full order of magnitude
more likely among the mentally ill than the general population.
I actually think the better conclusion is that we need better understanding of mental illness, care for people facing difficulties and removal of the stigma involved in seeking help. Also a wider social change away from the glamorization of violence and the constant capitalist message of inadequacy, but that's a bit heftier goal :)
I'm not sure how you can look at someone saying "Please don't assume that violence must be caused by mental illness." and conclude that removing stigma is a better
approach - "multiple murderers are mentally ill" and "mentally ill people are violent" are among the worst stigma, and lead directly to excessive institutionalization and victimization and unwillingness to get help.
When you use mental illness to excuse mass murderers, you are wrong
and you make it harder for some of the most desperate, marginalized people in our society to seek and receive the treatment they need, and easier for others to abuse and hurt them. This is a shitty thing to do. Please don't do it.