The Calling continues to torment him, the unnatural voices in his head battling his true human urge to protect you, and instead taunting him to shed blood. He thought he could fight it. But when he lost control and had you on the other side of his sword in a moment of frenzied rage, he knew he couldn't be selfish anymore. He had to face the evil that beckoned him head-on before he lost the most precious thing he swore to protect.
He was going to cast himself away into the nether until he could free himself from the control of the Shadow Lord. This was the only option, the only way in his eyes. He couldn't bear to live with himself if he ever hurt you, so he'd have to leave you instead.
That brings you here, sweltering heat waves hitting your face from the swirling portal in front of you. There he stood, eyes full nothing short of misery as he listened to you beg not to go. You could tell, from that expression on his face, that he was set on this path.