"Once upon a time, there was a boy. The boy didn't consider himself a sinner, but he was always trying to find ways to entertain himself. Eventually, he came across a group of other people who also sought entertainment. The group would find an enjoyable activity, get tired of it, and seek a new kind of thrill. They fell more and more from the Emperor's grasp, first into apostasy, and then into heresy.
Eventually, the boy, ever seeking new thrills, outright joined a traitor legion. He gleefully took to the battlefield against the protectors of mankind. He was promptly crushed under the treads of a landraider.