Yami Yugi did shit like let people light themselves on fire and die, send Kaiba to the card graveyard to experience death, mind-fuck them into hearing constant heartbeats, having everything they see censored or having watches grow out of their skin.
Sure he was still the hero punishing evil people, while Supreme King went full villain but it didn't even come close to the same level of dark as the early parts of the manga/season 0.
Judai is the better character/more interesting because of the arc he went on that led him to becoming the Supreme King and what he does AFTER he turns back. But he definitely ain't darker than Yami Yugi at his most brutal.