As a comparison...
Have you broke the speed limit lately? (Ok, that and stealing a car are different...but both are crimes) In doing so, let's say, you endangered my family which is driving along at the speed limit in a minivan (for example). You are even threatening my safety as a fellow driver on the road. Now, does that give me the right to shoot out your tires so you slam into a tree? You are breaking the law after all.
Ok, now take my family out of the picture. Now I'm just doing it to "protect" myself. Does the same thing apply?
Basically, what I'm saying is that I have no problem with bows/guns, in fact I go hunting with my dad in the fall. However, this man killed the car thief. This wasn't in self defense, he could have let the man get away and called the police.