'Another signee of the petition, signing under the name Sheila Zare, seemed to imply that students carrying concealed handguns should be killed: “If you're a student with a gun sitting next to me, you intend to possibly kill me. Why shouldn't I kill you before you kill me[?]”'
I think they meant "you possibly intend to kill me" which is quite different from "you intend to possibly kill me". In either case the belief is entirely and obviously unreasonable, and use of force would be unequivocally unjustified.