He's of the Old Republic. He started as a Jedi Knight succeessfully leading republic army in battles against Mandalorians all across galaxy. In aftermath of the war, he became a sith. Built a mighty empire but was faced with moral dilemma about the nature of the Force. He reformed as Jedi and tore that empire he built down (another Sith had taken over). Now becoming part of Jedi Order again was out of question for him as he found Jedis were hypocrites n no better. He had reach a state where he's able to use both light n dark side of the Force, transcending Sith & Jedi.