Gnolls are what happens when a hyena consumes the flesh, and along with it the soul, of a living human. Over a period of several days they change, gaining sapience and learning to walk upright. They can speak instinctively in whatever language their victim once spoke, as well as their own bestial tongue.
Even so, they remain animals. Cruel and sadistic animals, at that, who kill as much for pleasure as for food.
And it's not unknown for a burgeoning pack of gnolls to take living captives, solely for the purpose of feeding them to their still animalistic brethren.