to do something out of sympathy that will help someone.
An old man begged the soldiers to have pity on him and let him go.
have pity on (one)
To act compassionately or with sympathy toward one. Please, have pity on me, I don't want to die!The three spirits taught Scrooge how to have pity on his fellow man.
have pity on someone(or an animal)
to have compassion toward someone or an animal. Please! Have pity on us. Let us come in!