That's just the thing, you see. It's not the only possible definition. Other people have different definitions; QED. Nobody says you have to agree with their definitions, but it's not reasonable to just pretend they don't exist just because they don't jive with your own interpretation of what "Christian" means. The same is true for any named ethos, obviously.
If you want a substantive discussion on this — and for god's sake, please no — then it has to start with ideas, not bickering over semantics. Okay, fine, some people who self-identify as Christian do or say things that you don't think Christians should do or say. So what? They still do that stuff, and they still self-identify as Christians, and its not like anybody can sue them for trademark infringement for using the word without permission, so … what? What's the point of hauling out the "they're not really" thing?
Now. On the other hand. I could get tee shirts made up that say "I'm so Christian you just can't believe it" and then go on a killing spree and murder like a hundred toddlers. Would my actions say anything at all about Christianity? Not a damn thing, not any more than if I'd described myself as a Democrat or a Cubs fan. Unless there's some apparent causal link, or even compelling correlation, between two aspects of somebody's behavior, saying "That guy was X, so X is bad" is just fucking stupid.