negligent manslaughter is usually for acts of well...negligence...like you fly out of the back of my pickup truck cause I take a corner too fast or something.
every state in the country has some type of conceal-carry law, therefore possessing a weapon was not negligent.
if you could show that invader Zim was the initial aggressor then he'd be guilty of murder 2 (at least) but absent that showing you really have no case.