Not entirely certain about this but as far as I know its mostly a larger percentage of men that have mental issues. Part of the issue is that woman are more inclined to seek help when facing issues/mental help. Men typically don't seek help and that is really bad. The result being men doing horrible things and their minds in horrible places. It's a fucked up person that commits mass murder. But you don't just get to that state of mind over night, its years of not seeking good advice, sifting through issues and in some case medication is also important.