Source: comicvine.gamespot.com

5. Nobody

Henri Ducard was a professional manhunter and assassin. And while manhunting in Paris, he was approached by a young man in search of a teacher. That young man? Bruce Wayne.

Ducard trained Wayne alongside his own son, Morgan. But when Bruce realized the Ducards were big fans of murder, he ditched out on the duo. Henri sent Morgan to kill Brucie, but Wayne kicked his ass and took off. Morgan grew enraged by this and by the humiliation of losing to Wayne, so he trained for years to be better.

Eventually, he emerged as Nobody, a vicious killer trained in the murder-arts. He decided to not just kill the Batman, but to steal Robin away, as well. At this time, the Robin in question was Damian Wayne, who was struggling to control his own violent impulses.

Nobody captured him, tried to take the kid under his wing, and then eventually taunted the young Robin until Damien executed him.