My thought process is that he has spent the vast majority of his life killing animals for sustenance, seeing their life end, etc. I'm not a sociopath - far from it, but from the way I see it, as an outsider looking in, a person who has seen and caused so much death, if only to animals, would eventually be okay with the thought of killing humans if it is to protect his own life, or someone else's life. At least, they'd be okay with it faster than someone who has never killed anything. I can easily change it if that explanation doesn't make sense, though. ^^ It's all good.