One of the founding members of the X-Men, Warren Worthington III's time as a villain was short lived, but still a very significant moment in X-Men history. As a member of X-Force, Angel struggled with coming to terms with being Archangel (a persona implanted within him by Apocalypse years earlier when he regained his wings) and ultimately ended up losing his battle with the darkness within when that side of him took over. In the superb "Dark Angel Saga", Warren essentially attempts to become the next Apocalypse, forming a new Four Horsemen and attempting to destroy humanity. His girlfriend Psylocke ends up bringing an end to his reign, killing Archangel and leaving a new Angel in his place who has no memories of his past life. He's essentially an entirely new person, meaning that this original X-Man tragically ended up dying as a villain.
