It's not just one creation though. He wiped out Guinan's people. He tortures other species for fun. He's pretty hostile towards a lot of species. I guess they failed their trial. Picard must be one of the only defendants who won his case, and that's why Q helps humanity, while dooming other races.
They also make it very clear that if Q did anything remotely on the lines of wiping out an species, that he wouldn't be there anymore to bother humans. And that she did know Q from around the time the Borg shows up.
Star Trek is not really consistent. But if there's a backstory there, my guess would be that he watched and did nothing while bothering them all the time.