I alwasy kind of understood Kyle. I mean, after I realized that he lost Jodie (it does mention in the beginning of the...well, cave part), I just understood that he wanted revenge. On a smaller level he probably wanted to protect the new life he had found.
When he tried to kill Wanda I was pissed. I understood, but was pissed. She hadn't done anything. It was purely revenge that motivated him (in my eyes). And even after Wanda saved him he still wanted her dead! Trust me, I read the part where Jared punches him many times after that. Then when he told her that "a life for a life" thing, I was doubtful. Even when Ian said he was a man of his word. I was believing that he was waiting for the perfect moment.
Then little by little, as Wanda proved her worth, he starting coming around. He's really a nice guy living in a world that he's grown to hate. Think, if Mel had lost Jared ro Jamie what would she be like? I imagine something she would be something like Kyle was in the beginning of the book. After he found Jodie's body again, just the way he cared for Sunny, before and after they tried to get Jodie back, shows that he is a loving person, a caring big brother, and over all good guy.
Those are just my thoughts on him. Sorry if I spelled Jodie's name wrong. My copy of The Host is loaned out right now.