Revan saw people suffering and wanted to help them . the jedi council suspected the true threat had not yet shown and held back. he lead the younger jedi against the mandos and then discovered evidence of the Empereor. when he and his friend went against the emperoer they were overwelmed and corrupted. returning to the known universe they tried to conqueor it until malack bretrayed him and the Jedi purged him. that leads to KotOR where you control him and decide his morals