From what I see, Ralph was a gentle soul who didn't expect to kill anybody (there might be some arguments to this), and he didn't know he was going to kill these UK Aliens. So when he thought he almost killed one, he felt so much guilt that he wanted to redeem himself to feel better for killing something that looked like his own sister. Emily just took advantage of that feeling and made some kind of contract, and now Ralph is really unstable.

Can't wait until episode 12; I wonder how he's going (attempt to) execute his plan to save the other half of Emily.