Mordred is the bastard son conceived by Arthur when he sleeps with his sister, Morgause, by mistake. After Mordred is conceived, Merlin prophesies that the child will later destroy Arthur and his realm, and advises him to round up all the babies born on May Day and put them out to sea, in hopes of killing him. Before we even meet Mordred, then, we can't help but feel a little sorry for the guy, what with his own father trying to destroy him and all. But still, our feelings about him are mixed because at the end of the day he's a big threat to Camelot.
Mordred manages to survive the May Day massacre, but he remains something of a mystery for a pretty big chunk of the book. That is until an episode about a young unknown nicknamed "La Cote Male Taylé." In this story, Mordred acquits himself well and honorably by defending the young man with a mouthful of a nickname against insults from a lady with an acid tongue. So despite Merlin's prophecy, Mordred seems like an okay dude so far. Maybe this whole destroyer-of-the-realm thing will turn out to be hogwash.
Maybe… not. Mordred's character development takes a turn for the worse when he participates in the dishonorable ambush and stabbing-in-the-back of Sir Lamerok along with his brothers Gawain, Gaheris, and Aggravayne. One thing a good knight never does is stab another knight in the back. Plus, it seems to be purely out of spite that Mordred decides to aid Aggravayne in exposing Launcelot and Gwenyvere's affair. He has no stake in their relationship, so what other reason could he possibly have?
So maybe our Mordred's a bad seed after all. This is confirmed beyond a doubt when he forges letters claiming that Arthur has died in the invasion of Launcelot's lands, crowns himself king, and attempts to marry the queen. Not cool, dude. That's pretty much treason in Shmoop's book (and we would imagine it's the same in Arthur's).
With this, he adds dishonesty, disloyalty, and ambition to his list of un-knightly traits. He caps the list off with patricide – the killing of his own father – during the battle of Salisbury Plain. For Arthur, Mordred has been a living, breathing representation of his own failure in his lust for his own sister, and the one that eventually brings down his kingdom, just as Merlin prophesied. So the deal is sealed. Mordred is bad news.