Read this piece about why, statements of condemnation notwithstanding, the murder of George Tiller is very much a direct result of the actions of the mainstream pro-life movement.
While individuals who self-identify as pro-life may be well-meaning and against violence, mainstream pro-life groups and the people who run them do not care about life, before or after birth. And while today anti-choice groups are half-heartedly condemning Tiller's murder, they continue to use the same outlandish and inflammatory rhetoric that inspired and enabled it.
Words mean things. Anti-choicers should certainly have every right to express their views, but they must also realize that actions have consequences and their rhetoric is not harmless. If you yell "Fire!" in a crowded theater, it's reasonably foreseeable that people will panic and someone will be injured. And if you yell "Murderer!" "Baby-Killer!" and "Holocaust!" long enough, it's reasonably foreseeable that someone will take it upon themselves to make sure that vigilante justice is done (especially if you provide the name and address of the person who you claim is committing "genocide").