In the original Nanatsu no Taizai anime series, Arthur Pendragon was a good guy. But in Mokushiroku no Yonkishi, he's the main antagonist. What happened in the middle? Why did Arthur Pendragon turn evil?
In chapter 77 (Four Knights of the Apocalypse) Arthur states that he has come to the conclusion that historically humans have been puppets or collateral damage in wars between other races, and that, in light of this fact, true, lasting peace for humanity can only be achieved through an ethnic cleansing of all other races.
Another matter of conflict is that, in order to build his Eternal Kingdom, he is literally stealing portions of land from the existing nations and moving them to Camelot.
Arthur is shown to alternate semi-voluntarily between two personalities: one being serene and jolly, and the other being disturbing, ominous and ruthless. A visual queue for the switch are his sclerae being white or black respectively (as well as his demeanour and magic changing accordingly). It's clear that this is bound to the dual, highly polarized nature of chaos as was briefly described at the end of The Seven Deadly Sins. It's implied that this might be influencing Arthur's judgment, but the extent and nature of this influence has not been described fully as of chapter 127. It's worth noting that, despite visible changes in behaviour, Arthur appears to be of one mind in either state and, if indeed some magically induced personality disorder is at work, he has not shown signs of distress towards his psychological situation yet.