The mystery of Malak's jaw has never been solved. Most people think Revan did it, but none of the Biowarians has stated what exactly happened. I suspect it's there just to make him look more menacing. Hey, it's all fine, anyway. I love the way very little of Revan's background is revealed; it gives us fanficcers fun wriggle room. (In the unlikely case that you're interested, this is my version of what happened.)