If I recall these incidents correctly, they were loony liberals who went on gun rampages against other liberals in situations where they knew the would not face a hostile fire fight from armed self-defensers.
The people who'd walk in mass kill and maim totally innocent people in school, theater or whatever, are simply very dangerous mentally ill lunatics.
Religious fanatical terrorist, who do the same, also fall into this category.
But do not get me wrong, to be honest, I also happen to think that many leftist liberals are mentally ill, but not to the extreme of murdering innocents.