The Master Chief has massive psychological trauma from being a child soldier. That trauma has always been a central part of his arc, and whether the covenant's invasion justifies Halsey's actions in retrospect has always been a central question of the universe. The reason Halsey is kind of a Nazi is that she's implanted with a Geas by the librarian to follow in the footsteps of the Forerunners, and the forerunners sucked. Halo is concerned with big philosophical questions, and I like those questions, and none of the answers to those questions say fascism is okay.