The Hyenas want to eat, and that's it. They scavange for any scrap of food they can get their paws on. The Hyenas have always wanted to take over the Pride Lands, just because there is a lot of food in them.
Before Scar came along, all they lacked was someone who was brazen enough to plot a coup. The Hyenas are mean and extremely wasteful, but they were never as evil as Scar. They were just hungry.
The Hyenas are really good at being subordinate to the greater evil - they don't seem to think too well (or even at all) on their own. The only way they could ever take over the Pride Lands is with Scar's evil guidence. When they are betrayed by Scar in the end, they left quietly and tactfully, which is rather uncharacteristic of their species. For once, they knew when to quit.
However, I doubt they stayed quiet for long. I don't think one of those Hyenas could ever be full enough not to eat.