You were employed as an executioner by some group or organization in a position of power. Your legally sanctioned murders have given you a strong arm and a knowledge of death and fatal blows.
For some reason, you left your career: maybe you were wracked by guilt, maybe the group you worked for fell out of power, maybe you displeased your bosses, or maybe you decided to move on to a different occupation. But your actions have stayed with you, and your time as a dealer of death have made a lasting impact.