It comforts me that Tiller is burning and suffering in Hell.
IF YOU BELIEVE THAT CHRIST IS LORD, NEVER WISH THIS!
I'm comforted that God is in command. I'm comforted that God has infinite wisdom, far greater than that of any human being.
God, though His Incarnate Son, gave us "The Lord's Prayer". That states (in English, in the translation I believe is well agreed upon):
"And forgive us our trespasses,
As we forgive them that trespass against us. "
Note that trade: our trespasses will be forgiven only so much as we forgive other's trespasses.
I'm not going to lie and say that "asking for forgiveness for George Tiller" is easy. It isn't. His crimes against the most innocent were wretched.
But ultimately God is in command, and deciding who goes into Hell or who stays out is His providence only.
Here are two practical, useful reasons why we're supposed to forgive: First, it keeps us from allowing anger to eat us alive. Second, it gives us better ability to approach sinners like Tiller to try to help them. Or to honestly pray for them.
This isn't just theoretical. Some of the absolute best resources the pro-life side has are people who used to perform abortions. While we can debate if the actions of a theoretical (and regrettably not real) "reformed, pro-life Tiller" would balance out all the horror he committed, the truth is that this would be better for the pro-life cause than his not converting.