1) Because this is still the murder of a human being; even though the work that Tiller did was utterly and intrinsically evil, the goal of stopping him could never be used to justify this as a means.
2) Without a doubt, this incident will be used to legitimize the pro-abortion crowd's motives -- as others have mentioned, whoever did this might have made a martyr of him.
3) The most important reason: Whoever murdered Tiller likely took away his chance at repentance. That is the most tragic part of all.
Tiller had the chance to repent every minute of his life. As far as we know, he refused.
I'm not excusing the killer. But Tiller had a whole lifetime to repent. And if he did not repent, that was his choice. The Bible is pretty clear on this. Nobody knows the day or the hour of our death, except God. Therefore, it is unwise to go walking around in a state of mortal sin.
And that's also why Catholics ask Mary to "pray for us now and at the hour of our death."
God is merciful. He extends His mercy to all. I am sure that Tiller has had many opportunities to repent. And perhaps he did repent at the last. But for everyone, the time runs out, sooner or later.
If you were sitting your your couch cleaning your gun and Osama Bin Laden came to the door, would you be justified in killing him?
If so, then NEVER say Never.