In my father's last hours, he called my brother, my mom, and me into the room where he was laying and prayed that God would forgive him for all his sins, as he thanked God for all he had given him. He turned to my mom told her she was the greatest thing that had happened to him and would she please forgive him; she looked at him and walked out of the room. He died 15 minutes later. This act destroyed my brother and me, and it ended up consuming her life.
To have mercy on someone is to pull back from hurting them because you love your neighbor as yourself. You would not want a more powerful person to harm you, so you don’t do it to someone else. If a higher authority than King Herod had decreed that King Herod should be captured, tortured, and killed, Herod would likely beg for mercy. It is hypocrisy to be merciless toward others but expects mercy for oneself. The law remains: you reap what you sow.