A lot of these mass shooters probably would've been content to have peaceably ended their lives, but society said, "You can't do that; suicide is wrong." So these guys figured, "Well, if I'm going to break one rule, I might as well break a bunch of other rules, by not only killing myself, but taking a lot of people with me."
And those victims deserve it. After all, when did they ever write to their politicians to try to get suicide legalized? Probably never.
Yeah, some of the victims, e.g. in the school shootings, are little kids; but they would've grown up to be anti-suicide just like everyone else. They deserve to die for the same reason that Mexican kids deserve to be kept on the other side of the border -- because of what they'll be when they grew up.