Let's do it in a way that further emphasizes my point: Let's say a mentally unstable person kills a few people and the judge/jury, based on evidence, say that he's not guilty by insanity and then is sentenced to a mental institute. Now, let's say the institute says he's rehabilitated and, through various references and such, he's released or whatever the process is. Now, let's say he goes on to kill again, is it the fault of the Judge/Jury for not sentencing him to death just as much as the people that evaluated him?
Edit: BTW, it is the owners responsibility to do something about the dog and, in the case that they're unable/unwilling to, outside forces step in.