I won't disagree with you or Rick about mulsim extremists - they're a scary, fucked up bunch of people. They are at war with us. And, the world would be much better without them.
But, that logic of yours doesn't work for me. There are all kinds of extremists. For example, a person might be an environmental extremist who happened to grow up christian (but is not extremely religious). I would not call that person a christian extremist... more like an extremist who happens to be christian.
On top of that, it seems like a bit of a stretch to say that a person who is crazy is an extremist. I agree that mass murder is definitely extreme, but that doesn't fit the definition that I think of when I hear the word 'extremist'... especially if the murder is done out of insanity, and not in support of some cause. As far as I know, his reasons aren't known at this point.
To be honest though, I don't have a problem with people speculating about the fact that he did this fucked up thing, and that he is of a religion that, at the extreme end, is at war with us. But, I do have a problem people using hyperbole that implies that all people of that religion are connected to the acts of the wingnuts.