Allen pretty much said the reason why
"He cannot protect her from himself"
he is obsessed with Tia to the point he 1. joined the aristocratic group to plan and kill/harm her father
2. be a spy to put down the empire so Tia can be his.
3. Dreaming of strangling her with the ribbon he gifted her
4. Keeping her in a mansion away from everyone
He realized the harm he put her into when the duke planned the assassination that almost caused carsein's life. The duke's creepy smile frightened him to the point he realized what he did!
So he decided to confess his sins to the emperor and to the prince. Instead of being executed for treason, the prince decided to make him a delegate to rua since he believe in his talent and dedication to Tia.
C'mon! The boy understood his obsession and decided to treat his self.
He's a wolf in a sheep's clothing. His arc and redemption was wonderfully written.

I can't really say that this is a spoiler but:
Later tia gets stabbed in the future chapters and is on the verge of death. Ruve cries and begs tia to stay alive and keeps on telling her that he loves her and is crying.Since tia doesn't want to be a queen he even tells her that he would let her be the duke.
Tia survives and the first thing she does after she is run to ruve's office and their sincere confession happens and they kiss.
BUT WHY DID THEY MAKE POOR BABY ALLEN LEAVE!?!?! I AM CUTTING ONIONS!
(Btw they are taking a break because of ina has personal problem, I think but we won't get an update for a while T-T)