Mouzone is a Bishop because he moves down on Stringer in a straight line from the stairs, a Bishop can only move diagonally, not to mention the religious tie in here too. And if you look at the Wikipedia breakdown of moves a Knight can make you'll get a new appreciation for the scene.
Unlike all other standard chess pieces, the knight can "jump over" all other pieces (of either color) to its destination square. It captures an enemy piece by replacing it on its square. The knight's ability to "jump over" other pieces means it tends to be at its most powerful in closed positions (the unfinished building had no where for Stringer to escape), in contrast to a bishop. The knight moves alternately to light and dark squares (we first see Omar in the darkness and gradually see him move to the light when they kill Stringer). A knight should always be close to where the action is (pretty obvious with Omar), meaning it is best used on areas of the board where the opponent's pieces are clustered or close together (Omar always robbing stash houses).
juelz gif