If Arthur is the protagonist, that makes Mordred, the guy who steals his throne and then kills him, the antagonist, right? Bingo. That sounds simple enough, but there are actually a few other possible antagonists we could have named. Launcelot, too, plays an important role in the fall of the Round Table, not to mention the fact that he's having an affair with Arthur's wife. Other likely culprits are Aggravayne, who helps Mordred catch Launcelot and Gwenyvere together, and Gawain, who spurs Arthur on to war with Launcelot when he would prefer to make peace.
But these other culprits don't always mean to be so bad. Mordred, on the other hand, is the only character who betrays Arthur and commits treason and means to – there's just no other way to spin the whole coup thing. The fact that Arthur is also his own father makes his actions doubly dastardly. In the end, Mordred totally lives up to his name.