Alistair has his sword and shield at the ready, but he's walking backwards away from the creatures before he's paused to justify it to himself. He'll do that later: three Qunari and one Beleth are more than enough, surely, and he's already wrung-out from the blood mages and not any extra Templarish help against the shades. It makes sense for him to make sure the others' retreat is covered in case there's something more to deal with ahead of them. Later, when he's actually thinking at all, he won't think of this as a selfish decision whatsoever. Right now he isn't thinking, though, and he doesn't offer any explanations or apologies before he tells Detlef and Fate, "I have your backs," and follows them.