It's a simple enough metaphor for this particular, I think. After all, wolves and dogs are very similar: they're social animals, they can look very similar, but their temperaments to humans are very different. Their similarity is the reason I chose them as the metaphor. Good and bad people can look very similar.

As you said, a wolf may live in a forest near a town and never harm anyone. Yet the threat is there. A bad man may never act criminally but may still harm society: there are a number non-criminal ways to be considered a 'bad' person. They may cheat on their spouse, may drink too much, or any other thing that isn't illegal. This might not even make him socially unacceptable. That doesn't mean he is good. He is simply there.

On the other hand, a dog simply just be in the wrong place or time. There's been dog attacks. A dog may be aggressive towards humans or even other dogs. A theoretically good person may be forced into a situation where the only reaction is to bite. A lapse of judgement, the influence of substances, or even in self defense.

It seems that you seem to think that the guilt of a crime doesn't determine whether a person is good or not.