Now, with Magneto, you can argue that the man has the best of intentions. You can argue that he's simply afraid of his people being harmed. With Apocalypse, you can make the case that he's a godlike cosmic being who is just doing what's expected of him.
But Sinister is just a very, very bad and chaotic person. There's very little reasoning to really be done with him. He bas no honor, no real redeeming qualities of any kind. And yet because this one clone is a mutant, he gets into paradice while longtime allies like Captain Britain need a chaperone.