A proper view of self and sin is essential soil for the seed of mercy. Blindness with regard to one's own faults fuels the censorious spirit. A mote-hunter is judge, jury, and hangman. He dons the judicial robe and runs roughshod over mercy. Eagle-eyed relative to his neighbor's flaws, he is oblivious to his own.
Nothing promotes humility and mercy more than a reflective look at self.
Self-righteousness is an insurmountable barrier to the merciful spirit. Robed in Phariseeism, Paul drove merciless rivets of destruction into the church. David took Uriah's wife and life. Yet destitute of mercy, he hesitated not to judge and sentence a man to death for taking the life of a neighbor's lamb. Only by seeing himself and his sin was he able to reclaim the spirit of mercy.