Robert Diaz comments, "If mercy is the desire that the person's best good be achieved, then mercy is the very definition of love."
Which is true, and which goes to show we shouldn't expect too much in the way of precision from milk-drinking popular writers.
Still, mercy is an interior act of love, according to St. Thomas, a fruit of charity, according to the Catechism.
The mercy Mark has in mind, the mercy that is scandalous, is the mercy shown to those who have wronged us, the mercy we are to show because of the mercy God has shown us. In discussing mercy, though, St. Thomas quotes St. Augustine: "And what is compassion but a fellow-feeling for another's misery, which prompts us to help him if we can?"
Are these really the same concept? Can the object of this mercy be both a wrongdoer and a sufferer?
The benefits to the one who shows mercy to a wrongdoer are often pointed out, starting with the fact that we will be forgiven in the same measure with which we forgive others. But mercy is more than forgiveness; it's an act of love directed to the one forgiven.
So what is the good of the wrongdoer we desire when we show him mercy? St. Thomas writes:
It is essential to fault that it be voluntary; and in this respect it deserves punishment rather than mercy. Since, however, fault may be, in a way, a punishment, through having something connected with it that is against the sinner's will, it may, in this respect, call for mercy. It is in this sense that we pity and commiserate sinners.
When someone wrongs me, he causes me a material evil, but he also causes himself a moral evil, a lessening of his being. The resoration of this part of his being is the good sought by my act of mercy toward him. A sinner necessarily wounds himself by his sin, whether he realizes it or not, and Christian mercy seeks to heal this moral wound without first demanding the healing of the material wound caused by the same sin.
This is an astonishing thought. It doesn't merely outstrip taking an eye for an eye; it makes turning the other cheek sound unremarkable. It's like being punched and immediately offering to get some ice for the puncher's knuckles.
Temperamentally, I find it easy to forgive. That's my temperament, though, not Christ acting within me. Not holding a grudge, letting bygones be bygone isn't hard for me on a natural level. But on a supernatural level? Can I find Christ's sorrow at another's sin in my own heart?