In his mind, he is not able to blame himself for his life being in a bad spot.So he what, blamed Monique and her husband for all his troubles in life? Clearly this guy's life was spiraling downward, but why seek out the Tepe's in particular for retribution? Were there others he intended to kill, was he some sort of injustice collector with a list of those who wronged or slighted him? I may never understand why this man did this. Those poor little girls.
Here he finds himself, holidays 2025... he has no romantic partners, he looks like he was having trouble in his career (one that he spend a decade plus working towards), he probably didn't have many friends as he was socially inept. In short, he was a loser. But it can't possibly be his fault, it's Monique's fault! After all, she's the one who left him while he was in residency and if she didn't do that everything would be perfect.
^ That is his warped calculus. In his brain it's her fault that he is a loser. So she had to pay. And to him, the husband might as well die too because he has everything that MM couldn't have.