Cold fury
I have been more outraged by the brutal, senseless murder of Dr. George Tiller than by any number of atrocities in the world over the last several years. It cuts to the quick of what I believe in. His blood is not just on the hands of the killer and his immediate ideological allies, but also on hundreds of politicians' hands: the hands of those who have fought to marginalize and ghettoize women's reproductive health care generally and abortion services specifically. They created the climate that led to this killing. Many of them are, with whatever crocodile tears, celebrating it tonight. They are anathema to me.

As for his killers -- like you, I hold them all accountable -- I was a clinic escort in the 80s. I remember their so-called peaceful demonstrations.
(The Right might argue that the Left encourages terrorist attacks by suggesting that we ought to consider the needs of Arabic societies, but this parallel is too dumb to be remarked on.)
I don't think any civil or criminal liability can be attached. We hold people responsible for their acts regardless of their ideology. But in a purely moral sense, you can't act all shocked when you announce 'Abortion is murder' or 'God hates gays' day and night, and one of your listeners goes and kills someone.