Tiller was under investigation for not only failing to adhere to state law in regards to sending his patients to a non-affiliated licensed practitioner who also recommended a late-term abortion as being necessary to preserve the health of the mother , but for also for performing late-term abortions for non-medical reasons. Unfortunately, the first case against him was ultimately dropped (Sebilius is such a crook), but there was another one pending when he was killed.
You can read about it here.
I don't disagree with that, but what about being a terrorist?
Innocent until proven guilty, therefore it was murder, and even assuming he wasn't adhering to Kansas law (hah) it doesn't provide justification for murder since the penalty he would have face would likely not have been the death sentence, and furthermore theres the issue of vigilante "justice" which we don't allow, which makes this more like a lynching.