For the record, I have no sympathy for the Boston Marathon bomber. It was an inhuman and terrible crime that he and his brother committed, and he will have to answer to God for that. Cases like this can be difficult for those who thirst for justice and yet oppose the death penalty.
Myself, I subscribe to the writings of Pope John Paul II in Evangelium Vitae, that capital punishment is justifiable only as a last resort. It is a form of self-defense for a society, to be employed only when no other options would provide protection to its citizens. I do not think such cases are common in our world today; particularly in the First World.
The celebrations on Twitter remind me somewhat of the reactions to the killing of Osama Bin Laden. At the time, the Vatican released a statement to remind us that a Christian never rejoices at a man’s death.
Few tears will be shed for Tsarnaev, and I do not argue that it should be otherwise. But let us just remember that Christians are supposed to pray for their enemies. When and if his sentence is carried out someday, after all efforts at appeal are exhausted, he will see God and Justice with a capital “J” will be done.