I think the right principle is "reasonable force". The idea is not to protect scumbags - they can all go to hell, IMO - but rather to protect both sides when its not so simple as good guy-vs-bad guy;
...when I was much younger (around 10 or 11) a friend of mine (and no, this is not a euphamism for me!!) told me that he once broke into a neighbour's house when he knew they weren't going to be around. He didn't take anything. He didn't damage anything. He just did it for the 'buzz', because he didn't like the guy. Nothing more came of it. It was a pretty stupid thing to do, and in no way am I saying that it was OK to do even that.
However, what if the neighbour had been at home, and caught the guy. Would it have been appropriate to use physical force, just because you saw some guy in the dark? I think there are no easy answers in this situation. I've been on the victims side aswell, and nothing eats at you inside like being violated in that way.
I have no time for scumbags, professional thieves or anyone who would use indiscriminate violence against someone, but sometimes its just a 10-year old kid who's doing something that's pretty stupid, but not that malicious.