1 X: I am here to put humanity on trial for all its sins! By trying you, Captain Paris, as a proxy!
2 Paris: That… hardly seems fair.
3 Paris: Why don't you put, say, the sparrial race on trial, by trying Serron as a proxy?
4 X: Oh come on, that'd be no fun. We all know he's guilty.
4 Serron: Hey! I've paid off plenty of judges in my time!

It'a a standard alien trope - surely you didn't think we'd go much longer without doing it.

This is the real reason Stephen Hawking is afraid of us making our presence known to any extraterrestrial species out there. With his superior mind, he just knows that this is the sort of thing that will happen.