Hungry and eager to please, they wind up as Scar's evil henchmen when he promises them a meal ticket. Scar manipulates them to do some pretty awful stuff, and the hyenas obey. Hey, they've got to eat somehow. According to Shenzi, they're all "dangling at the bottom of the food chain."
Unlike Scar, however, the hyenas aren't evil through and through. They begin to lose faith in their leader when he fails to deliver on his promise of a bountiful, food-filled future. By the time Scar tries to pin the blame for the ruined Pride Lands on the hyenas, they've had enough.
In an ironic and fitting twist, Scar is devoured by his very own henchmen. Although by then, they're more arbiters of justice than they are henchmen.