It was caused by the addiction of power. The gunslinger, wielder of the sun, corrupted by, ironically, the darkness. The power flowing through him causing him to itch for more and more, his mind being consumed by the temptation. He studied and studied, trying to unlock the secrets of the sun he had began to control. But eventually, he learned of an ability that would allow him to reach his full potential but because of his lack of experience as a guardian, he was consumed, his body disintegrated by the power that would one day be used by many that fought to achieve it.