Bane was not always evil, but events in his life lead him that way. He was a member of a race called the Ancients, who existed long before any other race had even been born. Most of their civilizations existence was a golden age, almost a utopia. But something went wrong.
They started dying, with no virus or war to explain these dark events. In desperation a council of war was called, and it was decided that they should search for an answer to their problems. One member of the race, who was Bane's mentor, was tempted by the Devil: He would give the Ancients immortality, but first he would have to conquer the universe. Blinded by desperation, Bane's master accepted the offer. What followed was one of the greatest wars ever seen: The War of the Ancients. If they were dying before, this was accelerated by events. Although the forces of light won eventually, by then their fate had been sealed.
Bane was presumed dead after the last battle, but his spirit lived on. As for his mentor, he became one of the most hated figures there was: the Precursor.