Domestic Terrorism In The U.S.?

This morning an American doctor who performed "late term abortions" was shot and killed while entering his place of worship. The suspect in the murder is in custody now, and some ultra-conservative voices in the U.S. are subtly applauding the act, while those on the left are denouncing it as "domestic terrorism."

The question this raised in my mind is this: what's the difference between a fundamentalist Christian who does such a thing to someone he believes is a "murderer", and a fundamentalist Muslim who flies a 747 into an office tower filled with what he believes to be "infidels"? Both have strong religious beliefs that they're willing to kill and/or die for... so if we call one a "terrorist" then shouldn't we call them both that?

I can see both sides of the pro-life/pro-choice debate, and believe that it's a very tough issue that has no easy answers. But also I think that there are Christians out there who would do well to understand that the passion that they feel about the sanctity of the unborn is matched ounce for ounce by those of other religions who see their own belief systems being challenged or even trampled under. If violent reaction to that sort of perceived threat is "terrorism" for the one, then shouldn't it be "terrorism" for the other, as well? Or is it only terrorism when it's done for a cause or set of values with which you don't agree?

Like I say: no easy answers here.