If you want to get away with a murder, you get an illegal gun, and usually one that doesn't make a lot of noise (handgun + silencer); which is probably easier to do in the USA than in other countries since there are more of them around (though I have no knowledge of this).
If you are psychotic and you don't care about culpability, you order an assault rifle online or you go to a local gun shop and buy one to murder many.
Both acts are heinous, but one is probably more preventable than the other.
It seems like we could at least restart the discussion (again) on banning easily-modified semi-automatic rifles. But all legal routes to liability are generally blocked by the NRA, including going after ammo manufactures, taxing semi-auto assault rifle ownership to great extremes, suing semi-auto assault rifle manufacturers, better enforcement/banning modification of semi-autos, higher penalities for owning automatics, etc. And I seem to recall reading that even most 2nd Ammendment-supporting Americans are in favor of more gun control--the exact opposite of what the NRA wants, since their motives are profit-driven by manufacturers and not by their membership.