While I've no argument against that (aside from a few legal nipicks about how those are sometimes administered), I don't know how to expect even very harsh enforcement of such laws would have any effect on situations like this one, or the one in Vegas.
None of these guys can have much realistic expectation that they're going to live through it, or very long after it. The common expectation appears to be a violent end at the hands of responding police or -- very likely -- their own hands when they decide the event is over. Fear of arrest or imprisonment seems irrelevant.
(And if some do manage to be delusional enough to expect they'll escape after a mass killing, I'd expect they'd be delusional enough to expect they'll escape arrest and imprisonment as well.)
Zero tolerance laws about felons with firearms may actually have some impact on average street criminals (I really don't know) but these one-in-ten-million crazies seem immune. That is...if they even had a criminal record to begin with.