I think I can confidently say the break up is due to Maria wants to shoulder the burden of taking this revenge alone. She doesn't her hubby to go down this no turning back path as well. (I know, theres more to it, I'll get to that). Sounds familiar?
Kiri-kun also chooses to shoulder his burden of being bullied alone. Not wanting to worry his parents. I know this seems pretty generic but its interesting to see 'like mother like son' parallel in this series.
More accurately, the reason being is he didnt feel 'safe' to talk such topic with them. We can also use the term 'confident', 'trust'
And in Maria's case with his hubby, if we look at early chapters, we can see him telling Maria to stop digging. I believe thats the point where Maria lost her 'trust', 'confident', and or 'safe' feelings with her hubby. If only he was there for her, if only he was willing to listen to her, if only he was using his detective brain (well this one is asking too much, dude was grieving) would he be in the dark alone now?
I wish to see him using his brain on this case. But I really hope hes not going to be the final obstacle or boss or something. Wouldnt it be interesting see him actually the one delivering the final blow to the last bully, taking a piece of Maria's burden, shouldering it together?
Thank you for the sl!