I'm not sure I agree that she didn't seem bothered by having to kill him. I know that over the course of the show it's been Philip that's been shown as being very affected by having to kill innocent people, but I don't think Elizabeth is unaffected (and this is quite apart from the protracted death scene with the old lady in season 4 (was it?) that still feels to me like it snuck into The Americans from another show altogether). She's businesslike for sure, but I felt she was quite rattled by having to kill the young marine in the previous episode and that she always is. It's just that she's much better at compartmentalising things than Philip is so these actions don't haunt her the way they do him.