I think Jack's motivation is to save himself here...and Sayid hasn't actually shown that he 'likes' to kill people. it's always presented as a necessity for the context of the storyline.
Ben has been telling Jack that everyone on the island will die if he doesn't come back to save them, and saving people has been Jack's primary role throughout, starting from his ex-wife (and including saving ben himself with that surgery).
As for sayid, I agree they have shown him struggle from time to time with his violent "duties" but on the other hand it seems like that is just what he does and he knows it.
