If you look at it closely, you may have missed the part of where Ruve started to change and matured. I'm not justifying what he did in the previous life, but I feel like Ruve in the past was an immature and insecure kid for Tia's achievements in life. This was also kind of explained when the previous Emperor (Ruve's dad) was dying or died. It was because many people loved Tia and had always compared her to him — which is what he hated. The nobles and even his father was always praising her and he was always trying to beg for the same love and attention. Because of this, his love for Tia was buried with his hate and jealousy of her achievements. When he realized he loved her all along (set in the previous life), it was 4 years too late as he had already killed her.
However, this Ruve in the present life was a mature and well mannered kid. Tia unintentionally taught him to be so. That's why when Allen's father compared them (Ruve and Tia) again and Ruve only smiled, he thought the crown prince (now emperor) had matured. Ruve even says if he wasn't able to open his eyes, he would've been such a brute and immature person (what Ruve was like in the previous life).
P.S. It is later revealed that his father didn't praise him that much because he wanted his son to grow up into an independent and mature kid because once he (dad) dies, Ruve would have to stand up for himself in the lonely Palace. Bad parenting but that's how the previous Emperor thought was best.
Yeah I get what you're saying. How you are when your an adult heavily depends on your experiences in your childhood, and it's hard to get a proper and loving childhood in this time and place, with all the nobles and shit. Though I won't forgive past Ruve, I think I can just understand the present Ruve. However I don't want Tia to become empress and never become a knight, like she spent a whole other childhood training with her weak body to learn swordsmanship...
I don't want Tia to throw away her many years of work to become a knight just for some dude who killed her in a past life and gave her trauma. In her last life she lived and worked hard to become someone worthy for him and that went shit and now she finally worked and trained hard for herself and to form a path completely different from her first life, but now she's throwing it all away AGAIN? This story does not portray a realistic story and you can tell the author just threw in some trauma and misfortune just to make them fall in love again so it could be "romantic"