The relationship is a little creepy. She's a bit violent, including punching him at one point. He is over-the-top jealous and stalker-y, and a couple of times physically restrains her to get her to listen to him when she doesn't want to. On the other hand, we eventually understand why he acts the way he does, and when she calls him out on the creepy psycho stuff, he realizes that it's a problem and tries to change it.
Other than that, I think it's a good story. I have a soft spot for stories about beautiful bad boys who are head-over-heels over a girl (Beautiful Disaster, et al). The author does a decent job of showing the problems with dating a violent boy with stalker-ish tendencies and socialization issues, and how their relationship evolves as they learn he starts to learn how to act right. She also has some issues with defensiveness and not really feeling comfortable with her feminine side, and with his help, she starts to feel more comfortable in her skin.
The story also has a fair amount going on. There's the serial killer sub-plot, that is mostly about them learning to understand and trust one another for most of the story, but involves some suspense and action here and there. There's also a solid arc about the relationship between Ellie and her sister, as well as a strong dose of tomboy-gets-a-makeover story line. For all that there's a lot of potentially heavy things going on, the general tone of the book is actually pretty light-hearted. It's a fun YA read.