
.... He was awful to Tia waaaaaay before Jieun came into their world! He ignored her, purposely let her fall on the ground when they first danced to ridiculize her before every noble in the assembly... He didn't need to be forced into it. And what kind of poison make someone abuse specifically one person only? If this is the case, it would be a very convenient and very atrocious plot decision.

Nothing in the side story written in Ruve's point of view show that he was out of his mind because of a poison. He understood with perfectly clearity what he has done and he his remorseful about judging Tia without properly listening to her side of the story. He is shown to be a perfectly sound of mind asshole. When he saw just how hard it was to actually rule the country, he took his frustration on Jieun and used her as his new punching-ball.

I think I just understood, you were not talking about the first timeline but the second one, is that it? I thought that the one who is going to be poisoned by Jieun was Tia. Was I wrong? As for the raws of the novel, here you go. http://comico.kr/novel/titles/285/chapters/226#viewlst
.
.
.
.
.
.
.
The prince was under the effect of poison which made him do all those horrible things to Tia. The poison was given by Jieun under the orders of her caretaker who is the main villain. There is a huge conspiracy involved. The villain wants to have the throne for himself.
So all this time Ruve was innocent. He was always under the effect of poison. So he in general DOESN'T DESERVE THIS HATE. Because he is also a victim like Tia and unlike her the fandom ridiculed him.
Moral: Never judge too early.