You know, rereading this, I think I get why he took their engagement and her love for granted.
The thing is, they were under a political engagement. They didn't get engaged because they were in love with each other. It was one they were forced to take in. While she loved him, he likely never saw the joy in such a thing because it was something that deprived him of his freedom. Very few people ever like arranged marriages. It's something that restricts people's freedoms and he didn't even get to look for candidates for his political marriage. It was all decided. He likely resented that and thus just gave her the cold shoulder.
Should he have paid attention? Definitely.
Is he stupid? Yes.
But it's not out there for people to realistically be idiots and actually let their resentment and bitterness make them give someone that represents their restricted freedom the cold shoulder.
As for these letters, I think it oughta be clear that these letters NEVER made it to Wistash. It seems like Wistash never received any of these letters, so this is his first time reading them.
I don't like him, but I am wondering if he, after needing to be knocked down some more, will actually learn to be a better person. Even if he's the antagonist, I hope it's not like the horrible,
"I will do absolutely horrible shit to make your life miserable just so that you will come back to me and the only way to stop me is to kill me!"
I'd rather it be the case of,
"Please come back to me! I realize now how wrong I was, I'll be better! So you won't come back... okay, then I wish you well. I will never make the same mistake again and now I will be a better man."