I'm only modestly brilliant. "Amazingly brilliant" is ten points up the IQ scale. lol
"Mercy" does not mean "let everyone do whatever they want with no consequences".
If you steal my car and then beg for forgiveness... I may show mercy and forgive you. That does not mean that, as a society, we allow people to keep the car.
Did you take a Dale Carnegie course this semester?
How do we simultaneously both do justice and love mercy with respect to the death penalty?
We remember the victims of the killer as well. And their kin. They need mercy as well.
But to "love mercy" for a convicted killer (truly guilty, I shouldn't need to add), we need only share with him the Gospel before he dies. Let him know that he can still be loved and find a place in God's Kingdom, if he only places himself on God's mercy.
As for the "strange tension" there are two schools of thought here. First, and the most obvious, is that God's demand for "justice" was satisfied by Christ on the Cross. That is why Christ has to be God, to satisfy the infinite requirement of justice.
Secondly, and more specifically Catholic, we believe, like Paul that we "make up what is lacking" in Christ's suffering. Our own sufferings are God's way of using punishment in a merciful way. This grows us into the people we need to be to see God as He is.
And of course, the demand for justice can be accomplished for a killer about to be executed with his stay in Purgatory. In this way justice is always served.
SD