In 2007 Operation Rescue produced this video (warning, extremely graphic content), which doesn't quite call for the assassination of George Tiller, but it comes close. (Here is the link to the video on the OR website, where they take credit for making it.) The video opens by calling Tiller a "Murderer, Liar, Profiteer, Drug Addict, Corrupt, Unethical, Perverse, Alcoholic, Drug Addict, Malicious, Evil, Defiler, Foul, Blasphemer, Butcher." It ends with "Together, we can put an end to George Tiller, Abortion and these horrific crimes."
It is probably, as with abortion clinic bomber Eric Rudolph, that Scott Roeder had material support from pro-lifers in his long campaign to assassinate George Tiller. It is certain that he was actively encouraged by the most prominent pro-life groups in the country.