Compassion leads you to have mercy, which is like forgiveness. If you have mercy on someone, you let them off the hook or are kind to them somehow.
This is a quality that has to do with compassion, forgiveness, and leniency. If convicted of a crime, you might plead for the judge's mercy, meaning a lesser punishment. When people say "May God have mercy on me!" they're asking for forgiveness. Mercy may be given or received. You might show your little cousin mercy by not forcing him to eat a worm. People jokingly say a bad singer is showing mercy if she ends the show early.