He fully believed he was doing what was necessary to save humanity: erasing both impending fate and the mistakes of the past, and making a completely fresh start for history. Perhaps he thought he was allowed to have that one selfish thing with all the blood he soaked his hands in for his perceived greater good.
[She gives a shake of her head at nothing in particular, mouth drawn into a perfect line, before glancing back to Hei and studying him for a silent moment] I'll never know the thoughts, feelings or desires that he kept hidden. I can only guess. It's all I end up doing when I think about him, wondering "why". [A small, humorless smile]