I do not particularly mind the Diviner getting marooned. It's clear Tars Lamora was set up so that he wouldn't suffocate to death despite the ship at the heart of the mining operation now having disembarked, so there's probably enough automated equipment set up to ensure he survives. I'm also not really sure what else they could have or should have done in that situation either. They're not in Federation space, this isn't Starfleet's jurisdiction; making him subject to Federation laws and penalties and taking him captive isn't exactly justice either. And there are no actual Starfleet officers on board the Protostar legally empowered to do the legal business of arresting him for his crimes against Starfleet specifically. They can't let him go home, since he's a time traveler, and his people haven't even been contacted by the Federation/don't know alien life exists yet. Like, there's no real good solution here. And I think it's also probably important to remember that implicit here is that the Protostar-kids likely protected his life from reprisals from The Unwanted. You're gonna tell me, a violent uprising of slaves wouldn't have lynched their captor if given the chance? They did a lot more for him than they probably should have or were obligated to. Also, there's like, a 0.000003% chance we'll actually never hear of this character again either. They'll be back eventually for sure. If only so at some point, Gwyn can show him he was wrong all along.