First of all, if we started referring to people like Dylann Roof as terrorists, it is highly likely that a current of opposition would arise among black writers and non-blacks arguing that to call the killing of nine black people in a church “terrorism” detracts from the key fact that the murders were driven by racist animus. There would be accusations that the use of the word “terrorism” was even deliberately intended to distract from the importance and persistence of racism in American society. We call Roof’s actions a “hate crime” out of sense that the enlightened American is especially vigilant against, and appalled by, racism—this is part of modern educated America’s DNA and we should be thankful for it.
Perhaps a better solution would be to call both Muslim and non-Muslim murder sprees “hate crimes?” Yet, I suspect this wouldn’t go over very well either in real life. Part of why we don’t spontaneously label the actions of the Tsarnaev brothers a “hate crime” against Westerners is because our intuition is that their “hate” stemmed from something understandable on some level and likely traceable to American actions abroad in the past and/or present. Many would consider is outright dismissive to call Muslim’s anti-Western sentiment “hate” in a judgmental sense. Plus, in our hearts, by “hate” we mean hate against particular groups of people, and Westerner feels too broad a category to plausibly be considered a “group.”
The only other solution would seem to be to stipulate that racist murder sprees be heretofore termed “terrorist hate crimes,” while Muslim murder sprees be simply called “terrorism.” This would be hopelessly clumsy, with endless misapplications and provoke angels-on-the-head-of-a-pin arguments over what label is appropriate.