Even in the Federation, I'd say rehabilitation does not allow for full freedom once someone's released. It's possible his release would have conditions attached: No having any command of your own; No playing a role higher than bridge officer or Chief Engineer.
The man has actually committed genocide. So if he is given the death penalty or life imprisonment in our time, rehabilitation in the Federation would have riders, I think.
All that would be Starfleet, not the Federation; rehabilitation shouldn't ever have riders, because if you need riders then you're not rehabilitated yet and should just be kept in and put through rehabilitative processes until you are. There might be a stage where you're eased back into normal life under restrictions, but that would just be the continuation of a sentence with a potential for eventual end should you continue to prove yourself a changed person. Saying that someone should ever just be hit with a guaranteed-permanent situation like that is still a punishment-oriented perspective of criminal justice, that this person is bad and deserves to be hurt because he is bad.
And while he did commit mass murder, he didn't commit genocide which is a much worse crime; that might seem like legalistic or semantic quibbling, but the word "genocide" doesn't deserve to be diminished.
What you're saying is not nuanced. It's not that the person is bad forever. It's about how much power are you willing to put in the hands of someone who has committed genocide. How do you know that someone has truly rehabilitated until you put him in that situation again? Rehabilitation for common criminals is easy. You do your "rehabilitative processes", you hear them speak the right way, make the right noises. You put them back into society and watch whether they steal, forge or kill someone again. Are you willing to put this man in a position where he can kill hundreds of people again? And once he does that again? "Oh we need to improve our rehabilitative processes." I'm sure that's reassurance to the families of those killed because you let a former maniac loose again.
Reasonable restrictions make sense according to me. He would still be free to live any way he wants to. Just that he would not be in a position of power to kill hundreds of people on his whim or due to his emotional weaknesses.
And what's the difference between mass murder and genocide? Number of people killed?? It's abhorrent and deserving of extreme punishment either way.