Right, not typically, but mass murderers frequently do, in fact it's typical that a mass murder /massacre ends in the death of the mass murderer, whether by his (they're only males) hand or someone else's. Boulder is an exception that proves the rule, rather than typical of these infernal things.
It's so typical of mass murder /massacre to end in the death of the perpetrator /murderer, that we have to conclude that their basic plan is to kill themselves, and to take as many innocent people with them as they can. It's horrible, and cowardly. One of the only things I can think of to even have a chance to defend yourself against such a thing, is to have on yourself already a rifle or rifle-like firearm or ranged weapon.
When they do survive, it would be nice if our mercy shown to them is valued more than it is now. Breaking all the major bones in their body, making sure that they don't expire too soon, to maximize their pain and torment, is what they deserve for such treachery. We don't do that, but we are showing them mercy. It's not because they don't deserve it, or because we wouldn't be justified in doing it, because we would be justified, and they do deserve it.