I've been thinking what could be the relation between Nao-chan and the Nao from the Library who Sensei perceives as fear itself. So here I come with another theory straight out of my ass. Mind you, I'm not even endorsing it 100%, just having fun.
Akira was not always opposed to having kids, when he finds out Sekai is pregnant of his child he's actually looking forward to it. But there's a problem: his brother. The one who has everything he wants, the woman she loves and, if he doesn't do anything, will have his first child too. That is Akira's biggest fear, Nao.
There is where he draws the line and decides to kill his brother (not an uncommon scenario in here). It's confirmed he's been part of the accident, maybe even driving (does anyone recollect any dialog stating Sensei had a licence in the past? could be a clue), no idea how it went exactly. But he leaving unscathed from it is extremely suspicious.
Things go wrong and Sekai dies along Nao. In a cruel turn of events Akira is the one who ends fathering the other's child. Now Nozomu seeks revenge, he wants Akira to impregnate all the girls only to strip him from them. Send them to another plane of existence to work for the Gods.