It seems I have to be online every time people start saying blasphemies regarding Sparda's past.
Sparda was no softy.
He was defeated by Mundus when he and the rebels waged a war against their own kind. In the second confrontation, Sparda had the upper hand, and confronted Mundus alone. Then he did that ritual we all know.
It is unknown what changed Sparda's heart, but probably he fell in love with a human female - that's the only reason a cold-hearted demon would turn his back to his own kind.
When he realized humans were not as bad as to wipe them from the face of the Earth, he convinced fellow demon warriors to take the upper hand on Underworld's ruler back at that time, Mundus.
Some followed Sparda because they trusted and believed in his principles and ideals, and some for their own interests.