It don't think it would ruin anything, since Shepard is quite literally not above making a deal with space hitler for the sake of a sufficiently important mission, but it would mean that Steve would have to be quite sorry about it afterward. It would be a hell of a conversation.

I have fulfilled most of the things I wanted to do in this arena, except perhaps something with De Winchester. And, of course, we can kill them whenever we want, we don't need no dice to tell us when. Let me talk with De's mun, and I'll give you a more definite answer, if you'll let me know how the idea with Thane goes down.