The largest massacre of Americans since the civil war
No guns involved.
in 1978, liberal-socialist Jim Jones along with his doctor Larry Schact concocted a cyanide/flavor-aid mixed to give to the members of the People's temple agricultural project. While it might be looked upon as a mass suicide, the children that died were most certainly murdered.
Maniacs will kill, no matter what tools they use. In fact, in this case they considered using guns to shoot people, but even with that option, chose poison instead, rendering access to guns as a moot point.