he knows that the mail was from the princess, considering that he knows how vile that mf is, it's just right that he stole it, as for the other mails, i agree that it's overkill, he didn't have to do that, but i stand with him for trying to limit anrie's interaction with the princess as much as possible.
he could always just... tell her? Come on, she's a grown woman. I mean, he doesn't even need to tell her the truth, just warn her "hey, this dude? Dangerous, drug trafficking murder dealer or some shit. Womanizer even. If he writes to you ignore him", just some lie and that's it, not that hard.
But ok, let's say that he really had no other option than to steal the letters and throw them away. The fact that he's not only removing the letters from the princess, but other invitations, and ALSO reading her overall mail before her, even her business stuff, it's just controlling and downright bad.
I think (and hope) that they're just showing this to make a point that he's so desperate for his life to change that he's just trying to take control of EVERYTHING around him, including things that he shouldn't control like that, and that's toxic towards his wife and abusive. I mean, if even the servants that have been with you for longer think that what you're doing is a little fucked up, to the point of wanting to talk you out of it and disobeying you, then what you're be doing must be wrong. Dudes using the parental controls on his wife's phone, for god's sake.
the ml is pissing me off so much omg, I really hate it when they get all controlling like that, I really want the mc to find out soon and make that man suffer and regret (I would have half killed him for locking me up in the house, but stealing my mail? Hell no I would have choked him The Simpsons' style, I wouldn't have sent him to the doghouse he would sleep in the FLOOR OUTSIDE. Kneel and cry and beg for forgiveness you prick) (yes I'm spiteful like that)