Yes, his behaviour was unacceptable and definitely shouldn't be tolerated. However, it was simply an accident and nothing more from the sounds of it. I don't believe confining a dog to the garage for the night did any good for him, nor do I think your mom should euthanize him the next time he does something like that (IF he even does it again). I believe your brother is the one to blame for doing something Rupert that he didn't like. Not that your brother is a terrible child. He sounds young and probably didn't realize he was bothering Rupert. If Rupert goes after him again, I think it would be a better decision to rehome him with somebody without young kids rather than to euthanize.

Not trying to be rude but what's with the "I've been grounded" comment, anyway? I doubt anyone really cares. You say that constantly. This thread is about Rupert and whether is punishment was justified, it doesn't have anything to do with you being grounded.