I think it may be better to pray that Tiller had true sorrow to the Lord before he died and that God would have mercy on him. Also that his killer will also have true remorse and be shown mercy as well when he dies.
The only unforgiveable sins (by their very nature) are presumption...assuming that no matter what sins you commit, God will automatically forgive you. And the opposite of presumption, despair...assuming that it is impossible to be forgiven, so you do not ask God’s forgiveness because you do not think it is within His power.
And of course, not really being sorry for your sins would not warrant forgiveness.