The Quitter - S2.E5

Despite Darren, Wally, and Brick fighting alongside him, Arthur is effected by Blinder in battle long enough for Hammer to wound him and Blackwing to steal Excalibur. Wally blames himself for not being able to protect him from every harm, so moodily avoids the others - then accuses the others of blaming him for their own failure. Then he defects to join Morgana, giving her Merlin's wand.