I think it is as simple as Chris saw his Mother put down by his Dad and now just associates everyone with it and thinks they are all going to turn or die eventually. He keeps referencing how people turning is "just what happens now" and that seems to have really unhinged him.

I honestly think deep down Chris may look at what he is doing as an act of mercy that everyone else doesn't understand. Some warped logic like: "We're all gonna' turn if we die anyways so the only way to fix this is to kill people before they turn," or something like that.