Sombra wasn't born naturally evil. He didn't enjoy enslaving the crystal ponies that he was raised around. But he did feel different. He was the son of the Crystal Empire's King and yet Sombra wasn't a crystal pony himself. His 'father' made sure that he new that, and that he was a bastard child, and would never inherit anything. He was always in the shadow of his brother who was his father's true son, but Sombra never gave in to these shadows cast over him. Not until he was forced. But that would be much later. After he met the most amazing mare in the whole of Equestria.