When it comes to Arthurian mythos its about as complicated and messy as just about every mythos out there, so i wouldn't get too caught up on what a characters characterization is in certain variations of the story differing from other tellings. For example Mordred as a character wasn't even a villain or related to Arthur (hell in the very first instance of Arthur and Mordred as characters they died in the same battle but no mention is made of whether they fought alongside eachother or against eachother or any of that) and over the centuries Mordred variously is Arthurs child (like we have in this instance), nephew, archenemy and traitor, etc.