I don't feel sorry at all. If you want to contribute from there, show that the rogue professor and their students have been prevented from doing further malicious contributions (that is probably at least: from doing any contribution at all during a quite long period -- and that is fair against repeated infractions), and I'm sure that you will be able to contribute back again under the University umbrella.

If you don't manage to reach that goal, too bad, but you can contribute on a personal capacity, and/or go work elsewhere.