When it comes to Okaya, I hope his punishment will be the worse than the others.
I bet the fat guy will get betrayed by him and then Okaya does something that seems to have finished off Maria-sensei.
He thinks he won, then gets a message from his mum for dinner. Little did he know that Maria-sensei already approached her and manipulated her. The mother is deprived of love because the husband and son are absolute trash to her. The mum kills her husband, stash his body somewhere else. She puts sleeping drugs in his food. Okaya eats it unknowingly and ask where's father, she lies about it and told him the issue has been taken care of. He goes to his room but notice a small blood at one of the rooms. He doesn't alert mum and opens the door, to find his father's corpse horribly mutilated.
His mum notices and is sad by his son's discovery. The effects start kicking in and Okaya tries to escape and attacks his mother. The mum, of course, has the upper hand.
Okaya wakes up in front of Maria-sensei. His mother kills herself at home. Okaya tells Maria-sensei to finish him off already but Maria-sensei says it will be different this time.
Both of them will try to kill each other. If he wins, proofs of her killings will be awarded to him and he will able to rebuild his life back togethet since he will come out as the only survivor of her murders. If she wins, nothing happens and the case will never be solved.
We get a cool fight but ultimately, Okaya leads on. As Okaya injures her and about to kill her, the police arrived on time. Okaya thinks he's about to be saved and says he's got the murderer and evidence. Instead, he gets handcuffed and is told he'll be arrested for the murders of his friends and son. He is confused and tells them that they got the wrong person.
As it turns out, the game has been rigged against him so he was never going to win. Ex-husband planted evidence that will link directly to Okaya, making him taking all the blame. Yeah, ex-husband finds out it's his ex-wife and tries to turn her in but realises the police are pos so he betrays them and works with her.
The news paint Maria-sensei as the poor victim who lost her son to the murderer and went undercover to find out the truth while Okaya is painted as a sicko. His parents' death is said to be murder-suicide. We see Okaya in jail/asylum, apparently gone crazy. He won't be getting out of there in many years.