Mercifulness is a melting disposition whereby we lay to heart the miseries of others, and are instrumental for their good. We must chiefly be merciful to the SOULS of others. Indeed soul-mercy is the chief of mercies. The soul is the most precious thing; it is a rich diamond set in a ring of clay. Had we seen that madman in the gospel cutting himself with stones it would have moved our pity (Mark 5:5.) To see a sinner stabbing himself and having his hands imbrued in his own blood, should cause us to sincerely pity him.
That is a cruel mercy; when we see men go on in sin, and we let them alone. And that is a merciful cruelty, when we are sharp against men's sins and will not let them go to hell quietly.
Acts 20:29-31 ... For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock. Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them. Therefore watch, and remember, that by the space of three years I ceased not to warn every one night and day with tears.