What weapons are illegal to use in self defense of another?

Depends on the state, and really your differentiating the difference between use of force laws and weapons laws. For example, its illegal in Texas to carry around an illegal knife. Its legal to use lethal force in certain situations. If, lets say you were robbed, and were carrying a Bowie knife (an illegal knife in Texas. . .I know sad) and killed the bad guy you'd have two separate issues. The use of deadly force, which probably would have been justified, and the carring of an illegal weapon. The guy would probably would get a misdemeaner charge for carrying of Illegal weapons.