Seriously? You killed a serial rapist and a murderer, and you feel guilt? Makes no sense. The only bad thing about Nobuto's death was how quick it was.
He knows the guy and heard his story from the bar and hes not completely heartless. Even though Nobu was complete scum he killed his son afterall and felt immense guilt seeing his reaction. A fool, but hes not a complete monster.
Why the fuck did the mc go back to Reika's apartment though? He has literally no reason to be there outside of wanting the confrontation between him and Nobuto's father. I like this story but it's hard to suspend disbelief when things happen not because they have a reason or make sense but because the narrative needs them to to keep going.