We must've seen different Maxsons, then. His primary mission in this game is being anti-Institute, and sees synths as tools of their creators. It's a perfectly valid point, as not everyone has our privilege of having multiple synth friends. All the news from the Commonwealth is that the Insititute and Synths are killing and replacing people, Danse has personal reports that brought the flagship in to begin with... he's working with the very valid intelligence he gots.
But Blind Betrayal... he wants Danse executed for being an Institute weapon, and you think he's going to be the heartless zealot that the forums hate him for. Yet he can still see reason, that Danse is Danse, and offers him the mercy of exile.
You saw a poorly written character without a strong personality. I saw a strong leader pushing for the success of his mission, who was just given reason to doubt himself. Danse he let live, because even he knows he's not that heartless. And if he's given the benefit of the doubt to one Synth... if we do ever see Maxson again, I dearly hope they take a peaceful Blind Betrayal resolution as canon. There's an Elder who did what he had to do for the safety of mankind, but now has doubts and regrets lingering in the back of his mind. Does an older Maxson change his policies to repent for his necessary sins? Does he become a hollow shell, pushing his agendas when he knows there are innocents at risk?
Best part is that a Brotherhood ending need not be taken to get this far. He could live, and still leave room for other victories.