If our plea for mercy moves the heart of a man who cruelly murdered innocent babies, the angels in heaven will rejoice. But whether it produces that effect or not, we will have shown all who have eyes to see and ears to hear that our pro-life witness is truly a witness of love—love even of our enemies, even of those whose appalling crimes against innocent human beings we must oppose with all our hearts, minds, and strength. In a profoundly compelling way, we will have given testimony to our belief in the sanctity of all human life.
I do not myself believe that the death penalty is ever required or justified as a matter of retributive justice. Many reasonable people of goodwill, including many who are strongly pro-life (and whose pro-life credentials I in no way question), disagree with me about that. But even if the death penalty is justified in a case like Gosnell’s, mercy is nevertheless a legitimate option, especially where our plea for mercy would itself advance the cause of respect for human life by testifying to the power of mercy and love.
The less we support killing, the weaker the pro-choice movement becomes. Because if we won't even execute a killer, why should an innocent baby be executed? If we show mercy to a killer, why shouldn't the mom show mercy to a baby?