In my mind, he is indeed a domestic terrorist. He was affiliated with hate groups, wanted to start a "civil war" and killed those innocent people solely because their skin was black. He is the absolute definition of a terrorist. People didn't balk at calling Timothy McVeigh a terrorist, so I don't see what the problem is with calling him what he is - a racist, domestic terrorist.
You target a church, where you know people will be unarmed, and where it will have the most shock value on society - yes, you are a terrorist.
At 21, it's sad his life turned the way it did as he was so young that you would want to believe that he could have been turned around, but that kind of seething hatred and feelings of powerlessness turned into a malignant cancer in this young man. These same issues are what fuel terrorism wherever it occurs. We don't hesitate to call ISIS terrorists - we shouldn't be hypocrites.