I don't think that it's intrinsically evil to kill someone like Mr. Tiller with the intention of stopping the mass serial murder of such a person.
However, in looking at the likely consequences of the deed, that it will likely shore up support for legal abortion, that it will bring disrepute to the cause of life, that it will likely further harden the souls of many pro-deathers, that it will likely put off further the day when abortion is banned, the killing of the scumbag seems immoral because of the great evils it brings about.
There may come a time when a critical mass of Americans decides that the rights of unborn children will ultimately only be vindicated through violence. And then there will be war.
What will be done morally during such a war (including the field trials and summary executions of abortionists) will have been immoral before that time.
Actually, I'm not so sure. This late term abortion of Mr. Tiller may bring some attention to the kinds of crimes against humanity that were being done by him and others in the name of the Constitution and may actually cause some other would be mass murderers to choose to use their medical skills to save lives instead of taking them.
My cat killed a little mouse last night. I felt really bad about that. I could only wish that I had a heart that could feel bad for Mr. Tiller. In this case, I feel sorrier for the cat.