One issue I find is, when you accept faith-based thinking and supernatural claims as "normal" or "virtuous", how can you actually say that person is "not a real Christian"? After all, God killed all sorts of people throughout the holy bible, and even almost had Abraham kill Isaac. So, one could say, "hey, in some situations, killing is actually ok". What if that killer thought God told them to do so? Christians see "personal revelation" as a valid form of evidence, so he do we determine that he's actually wrong? Why isn't his version of the religion the "right" one? "Faith" is inherently vague and unreliable, so it's not like you can provide some sort of solid religious reason to oppose his actions. Sure, you'd probably say that Jesus wouldn't like that, but maybe that was just your "personal revelation". Which one is the correct one? Why would your "personal revelation" take precedence over the killer's "personal revelation"?
Now of course, you'd likely disagree, and probably use some outside standard that has nothing to do with personal revelation, faith, or god's actions. Maybe you'd make some sort of golden rule argument. Maybe you'd say that killing people causes pain, and being a human being capable of empathy, we should do things to avoid pain. Maybe you'd say that god doesn't actually talk to people (that's just crazy!) and tell them to do things, so that's an obviously silly justification for killing.
In that case, congratulations, you have a secular moral code. Welcome to the club
Now sure, some random atheist person can be a horrible killer as well. But how does he justify it? One thing we can determine is that "God told me so" wouldn't be used as a reason, so it at least makes this a little easier to deal with and try to prevent in the future. Unless he's just crazy, which of course can apply to anyone, and isn't some inherent feature of atheism.