Because, as evil as Tiller was, murder is murder. As noted in the post you were responding to, somebody lost her brother today. That is tragic.
The true tragedy is in the lost lives of those unborn children at the hand of Tiller.
While I won't celebrate, diminish or glorify his death in any way, Tiller's murder is not tragic.
George Tiller was a serial murderer. Now, someone has murdered him. That person has also killed their future and perhaps lost their soul to rid the world of a serial murderer.
I sincerely wish that Tiller had found the Lord, the Giver of life and repented of his serial slaughter of the alive unborn. But if he didn't, he will spend a very horrible eternity because of his chosen means to wealth.
The media spectacle of ginning up hate toward pro-lifers will be a circus for a while. Feminazis will exploit the murder for propaganda purposes.
But there are entirely too many Americans who will spend eternity in the same horrible end as George Tiller. Evil will use this murder of a serial murderer to drag even more souls to Hell by exploiting thetangle of evil that is abortion on demand in America and the twisted mindset of far too many who refuse to admit the alive unborn are every bid fellow human beings, thus they enable the evil to persist and grow.
It is an interesting philosophical question. Is it tragic when evil men are killed? Was the hanging of Mussolini tragic? Is the execution of a murderer tragic? Can it be tragic and justified at the same time?
This was an extra-judicial killing so it stands outside the law. It was a crime, certainly.