It is interesting that in all annihilation situations, there are feelings involved. SP, allegedly, "loved" AF. CW loved NK. And I think that either PF was genuinely attached to his child.
But all these guys, so different (because you can't compare gregarious SP with odd CW or this guy, from whom the most we heard was "get off my property"), have one major flaw. They seemingly don't understand that one just can't replace people and go on.
SP didn't like how Laci's body changed during pregnancy, he did not want kids, so he invited AF to go to him for vasectomy. Like, "I shall kill my pregnant wife, and live with this new woman and her kid, and have no kids of my own".
CW has a family, and the kids he loves, but he fell out of love with SW, and his new flame wants own kids, and - just like this! - let's kill the old ones and get the new ones with the new woman.
(He never asks himself, what if the new flame will be unable to have kids, or the new kids will have issues, be less than perfect. Then what? Kill them, too?)
Now I don't know the exact details of PF's life. People think there is another woman, but even if so, I won't be surprised if this is not even a passionate love. For him, it matters to 100% own the kid.
But even the simplest thing - how would it be for the child to grow up without the mother, or with a stepmother - doesn't enter his head? (He should understand the concept of motherhood, he is attached to own mother). How come he doesn't care about his child?