I don't like the way Reiji's acting now. I feel like not taking the deaths of his mom and Akira serious and giving them a proper farewell will bite him back later on. Like would've killed him to shake Akira's hand?
Also is he his father after all? I'm still very confused about who his dad is.
He grew to hate him because he realized him and Nagi just got used by him as ideas for his books, and because he demonized Yuuko. That, added to his hatred for his mom because he was just a stand in for Akira, you can say he's fed up with everything. This manga isn't called Shounen no Abyss for lulz.
I agree tbh, while Reiji's descent into his current form is arguably the "abyss" the series has been building to this whole time, something about it just feels off. I think I'm still disappointed with how Nagi's backstory was ultimately revealed, so having that be the final catalyst ruins the impact for me.
Also, I'm pretty sure his dad was always the man who raised him and who Gen killed. Yuko only told lies about his parentage to ensure no one else would love him, and perhaps also because she still wished in her heart that Esemori could have been the father of her child. The question of who Reiji's father is was completely dropped after Reiji realized this (i.e. after Grandma revealed the lie Yuko told her), so I think it's the intended answer. Reiji is only calling Esemori his father now to get around the restrictions for visiting hours.
As for my thoughts on the chapter: depending on how close to endgame we are, my theory is that Reiji is going to fail to murder Mao, Mao will escape and report Reiji to the police (and probably accuse him of holding Nagi hostage too, which is only going to look very likely with them using Nagi's phone to communicate), Reiji will run away and meet with Sensei, and Sensei will be revealed to have been the woman in the beginning of the series.
A thing that would debunk all of that: the suicide woman from the beginning has the bang covering her left eye, Yuri's hair covers her right eye and the ch1pg1 intro in the japanese mag version had an opening text that said 少年は愛する女性と光を目指して。。。 ("a boy aiming for the light, with the woman he loves..."). Yuri... is not that woman (Reiji despises her) and Gen also has his bangs combed to the right when he let his hair grow after the 1st timeskip. Only Nagi and Chako have the bangs in their hair covering their left eye, but Chako just cut her hair a few chapters ago, so that only leaves Nagi as the suicide woman, besides, her hair began to grow back to the length of the 1st chapters during the time Yuri is brainwashing her into confinement.
Speaking of Reiji's parenthood: considering Yuuko thinks it's the 3rd time Akira came back (2nd in her eyes was when he brought Nagi to the town), that alone is enough to set the alarms of Reiji's real parenthood.
The shit that can happen is that Yuri will stop an unhinged and unbound Reiji and he will end killing Yuri in exchange because he will be absolutely pissmad (paralleling a bit with Yuuko killing Uryuu). That will be Reiji's final abyss: finally understand he is just like his mom in everything, even being a murderer. And admit he should have not been that harsh with Esemori, giving him an actual reason to die that would parallel Nagi's.
Mao only wants to make sure Nagi is alive and after confirming it, will fuck off forever back to his selfish life (if he doesn't get killed first). That will be the last straw that will make Nagi realize Mao just used her, giving her a last wind to stand up and leave that basement, reach Reiji and pull the double suicide. Final chapter is gonna be the survivors commenting on the double suicide, possibly saying it's the result of horrible adults ruining their lives, and possibly throwing Reiji and Nagi's ashes into the sea.