This highlights how Catholic "Mercy" is a term often used as a polemical cudgel.

In any given moral situation, it's not really possible to extend "mercy to all."   That is, if mercy is given to one side, then the other side is diminished in equal measure.

For example:  extend mercy to criminals by putting them back on the street.   The law abiding public pays for that mercy by being victims of crime.

Extend mercy to 11 million people here illegally.   11 million here legally pay for that mercy by reduced access to jobs, or lower wages.

There is no "win-win" to this game of mercy.  There's always a zero sum result.