The above quote indicates that justice is a lesser virtue than mercy. This is wrong. There is a corresponding virtue to Mercy (forgiveness) and that is a cardinal virtue, justice. Without justice, there is no need for mercy. But mercy is not the greatest of virtues, as clarified by St. Thomas Aquinas in the Summa:
newadvent.org/summa/3030.htm#article4
The latter part above says we show the greatest virtue to our neighbor by mercy, but in the case of murder, the justice shown to God in taking the life of another dictates that the common good of all is of primary importance. On the issue of justice in the case of murder, St Thomas says:
newadvent.org/summa/3064.htm#article2
Mercy shown to the guilty by an injured party is not the same as mercy shown to the guilty by those who must take the good of all into account. Justice would be distorted in that case. The elevation of forgiveness above justice is contrary to the Divine law in which we participate by the natural law. They both exist in God:
newadvent.org/summa/1021.htm#article4
So justice is not contrary to mercy (forgiveness). As far as the death penalty as a human law is concerned, St Thomas says that human law can be changed:
newadvent.org/summa/2097.htm#article1
Pope John Paul II said the conditions of incarceration have improved so as to reduce the necessity for the death penalty to practically nil. This is not a dictate of law though, but of conscience. It is not against the dictates of the Divine Law as revealed to put the guilty to death, as stated by St Thomas:
newadvent.org/summa/3064.htm#article2
Human mercy elevated above the justice shown to God (as in the killing of those created in His likeness) is wrong. It is also wrong to use the death penalty as revenge, as well as to kill those who could adequately be kept incarcerated for the safety of society. One must examine themselves carefully as to their motivation for supporting the death penalty as the God who searches and knows our hearts will judge us on Judgment Day. Support for evil is also condemned as sinful.