Jesus, his reasoning may have been wrong but his judgment was still ironically correct. What an unnecessary episode. Better step it up next week Madhouse.
The shut happened because he didn't confront his wife properly and instead preferred to just keep it for himself until he blew up on the worst fucking moment
He is a little murderous bitch who just didn't went to hell because his wife preferred to sacrifice herself
But Machiko never had anyone on the side. There was no main squeeze. She lied to protect her husband which Decim couldn't comprehend.
The only memory he had of an affair is a one night stand post marriage, after Takashi had created the hostile environment of mistrust and suspicion. He didn't even entertain the idea of Takashi being at fault due to taking a confession at face value.
If you apply the qualifiers Nona spoke of when dealing with judgement then he was completely wrong.
She's telling him even though he fucked up don't dwell on it to the point it prevents him from doing his job.