[@Legion02]maybe he could believe that the king is going to destroy their kingdom? Such as bringing them into a dark time. His actions, to himself, would be justified because he believes he is doing what is best for his kingdom. When really, he's the one that is causing all the chaos. 

Or possibly, say he had a daughter or son that was once in a war controlled by the king. And his daughter/son was killed in battle. He could blame the king for the death of his only chil, And simply seek revenge