Clint has a soul mark. It is on the right side of his waist. It is a spider with a red hourglass. Clint later learns it is the mark of the Black Widow. So his soul mate is an assassin. When he is asked to eliminate her, he is devastated. But looking into her eyes, Clint saw a glimmer of hope. She is not beyond redemption.
Natasha gets her soul mark when she is 3. It is on her spine. It is an arrow with purple fletching. Natasha jokingly thinks that her soul mate is Robin Hood. Years later she hears of the infamous Hawkeye tasked to eliminate her. Natasha is tired by that point. So if she is to die, better be it by her soul mates hand. But he spares her and offers her a chance to clear her ledger. She takes it without a second thought.