The Master has been one of the Doctor’s nemeses for as long as we the audience have known. Though this Master is quite different from your traditional one. Whereas the usual Master tends to be your traditional mustache curling schemer, this Master has existed for so long that there’s been a shift in his viewpoint in personality. He is still a schemer, but he doesn’t do it for self gain or joy anymore. But instead because he believes this is just who he is now. The Doctor and him have battled for so long and through so many regenerations, that he believes if The Doctor represents kindness and goodwill, then he must act as his equal and opposite force.

Essentially, we're going for a much more cold and calculated Master. A man who's lacking his emotions due to millenias, or even eons, of suffering.