Terrorists generally don't distinguish between people - the word you are looking for is "assassin," I believe. Terrorists just want as many people to die as possible. They're more concerned with body count, take September 11, for example. As such, this guy that shot Tiller does not - IMO - fit the definition of a terrorist.
In this case, I would argue that if this guy killed Tiller because he hated Tiller, hated what Tiller did, and wanted vengeance for the evil that he believed Tiller wrought, he would be most accurately described as an assassin, not a terrorist. If, on the other hand, he attacked Tiller for all of those reasons, but also because he wanted to send a message to other abortionists/abortion-supporters (stop doing what you're doing, or else), he could fairly be described as both an assassin and a terrorist.