I think that's what we all wonder with family annihilators. It always seems to come from nowhere. I think it's a combination of a lot of things--I expect that while he was submissive to women (SW and his mother), he resented them (again, SW and his mother). He's probably never had to take responsibility for his actions. He probably also has limited emotions and ability for empathy. I think he's probably good at modeling caring parenting behavior without actually feeling it. Does that make sense? I would guess he modeled a lot of his behavior after SW's to make it seem like a good dad. Not actually feeling those emotions would make it a lot easier to kill those poor girls. I would guess that he also blames SW and in some ways, NK for his actions. In some way, he may feel that NK spurred him to do this because it was the only way they could be together.
This is an area of study with a huge amount of research and a lot of mixed conclusions. This is all speculation, but again, I think it's a combination of a lot of personality traits combined with events, like the relationship with NK and the conflict with SW and his parents.