Sean had not been untrue to himself, he'd just been his untrue self. He wasn't unkind to Martha in his mind, even though he'd left her with the two twins, he was just being kind to himself. In Sean's way of thinking, truth was only a temporary proposition imposed upon you by someone else, like a wife. In Sean's mind, kindness was something that you bestowed on strangers who could help you make money or get ahead. Everything was truly transaction, predictable and not unfriendly.
Love, to Sean was not truth, it was simply something that his parents told never really mattered. He'd learned in his parents' marriage that love was simply the kindness of tolerating each other day after day after the reasons had faded and the children were on their way. Unkindness would have meant living with Martha even though he lacked the kindness to pretend to love her, so he could not understand why she thought it unkind of him to leave her. Sean was being true and kind when he just didn't come home that day in London and left for Morocco and knew that Martha's stoic Mother Katherine would handle all of the chaos that followed after he closed the door.