More generally, boobytrapping stuff has lots of potential harm to lots of potential people. (gas company employees, police, etc). The fact that it's also harmful to thiefs is tangential.
That he was up to no good was irrelevant.
I once was ticketed for being at a park after sunset in our town. The cop explained to my friends and I that if we had just been across the street at the elementary school then it wouldn't have been a problem.
Actually, that depends entirely on which state you live in. Some states under Castle Doctrine allow deadly force to prevent the commission of any felony in your home, including robbery.
That, so far as I know, does not apply to booby traps.
http://en.wikipedia.org/wiki/Castle_Doctrine_in_the_United_S...