The King feels so incredibly betrayed here after he gave all that he had with love to his nephew and trusted him more than any other. He's acting rashly and hastily out of grief and passion, not able to understand how this could have happened or how Tristan could do such a heinous act. He feels that he has truly lost everyone close to him that he loved- the last of his family, the only wife he'd known, his most loyal of his court. He's now trying to fill that gap in his heart and the only way he knows is through what he believes to be justice. Because loving the gentle and kind people in his life didn't work out for him and betrayed him, he's now willing to love the "evil" and negative ones instead.