That's really the best word to use to describe the antics of the Operation Rescue (OR) crowd - the group that had been harassing Dr. Tiller and his staff for the last umpteen years for performing a legal service with which they disagree. Hilzoy at Obsidian Wings describes in some detail here.
The suspect currently detained was reportedly a follower of OR. He also reportedly had a conviction for driving with bomb-making materials. He was a fanatic - an extremist subscriber to an extremist philosophy.
The irony, of course, is that an abortionist was killed by a pro-lifer, be it the suspect in custody, or someone else. What shouldn't be lost is that this is the result that those such as Bill O'Reilly and Troy Newman had undoubtedly hoped for, even if they couldn't outright ask for it.
What this amounts to, essentially, is someone killing someone else because they have a difference of opinion. Dr. Tiller believed that he was performing a service to women who requested it, making a legal choice. His murderer, and many pro-lifers, believed that he was nothing less than a murderer himself, because their faith dictated it be so - their belief was, by this virtue, founded on another belief. A difference of opinion resulted in the cold-blooded murder of a doctor and obeyer of the law, yet there are those defending the action of the murderer as justifiable.