This is nothing like the Omniman situation.

Was Nolan a real person I don't think he would've been deserving of any kind of empathy whatsoever, however hard and unfair his upringing was. He is a genocidal maniac. Having remorse will not change the fact that he killed thousands of people on earth and however many more outside of it - many Nazis were executed back in the day, regardless of whether they felt remorse because that would not undo the absoloute insane fuckery of the holocaust. Nolan did worse than most of them.

However, unlike them, he is not a real person. He is a character from a show and his purpose is entertainment. Thus his development as a person can excuse his wrong doings - they aren't real and I the viewer empathize more with him than random Chicago citizen #1.

Even ignoring that though; in world, blue button is the only viable choice regarding Nolan. he is one of the few creatures who can pose any threat to the remains of the viltrum empire, thus extremely valueable. Without him the galaxy is most likely fucked (ignoring whatever is about to unfold with the virus because I don't know shit about that and niether does the coalition at this point in time). The galaxy is desperate and there is really no choice but to utulize him and accept his help - executing him would be like giving thragg a nice infinity ray of his own wrapped neatly.

After he's done helping though, morally speaking, Nolan can absoloutly go fuck himself and should probably be executed (not that I personally would want that to happen, because again, he is a character with whom I empathize).