Tia did not deserve her fate in the first timeline except for being "cold" and "aloof". That is the reason why so many people are calling victim blaming for the story, because of course she had to change to suit the tast of the ML better to get a happy ending... Right... The only "sin" she commited was using a knife against Ruve in the first timeline, and even that what completly understandable with everything she went trought and what Ruve was doing to her. She was pushed to do it by the poison the "real" bad guy gave her, but even without it everyone in this situation would have done the same. As for Ji-eun, a lot of people are defending her too, saying that she is one of the victim, being suddenly summoned to another world, away from her family and all. But the second timeline show that she deserve and deserved everything that happenend to her. The first timeline was Karma for what she did to Tia and the second timeline was all because of her own actions. She choose her future herself. For Tia's execution, she might not be the one giving poison the Ruve and Tia but she did push Ruve against Tia by lying to him and always complaining, so I hold her partialy responsible.
She was "true" to her feelings and said that she loved him, so he saw her as "pure" in a way. (but he realized too late that it was a lie and she didn't actually have romantic feelings for him, a little before his death).
In order to enable Tia to accept her seat as empress, I really feel like the black haired woman (Jiune was it?) has to come. To assure of the Prince's feelings, Tia has to really see for herself that he is really willing to only marry her.
Then again, I am curious. What caught the Prince's attention when it came to Jiune? I think his feelings of rivalry against Tia as he grew up has been there since her past life. They spent time together as children, did they not? Also, what did Tia do that deserved her that horrid execution? Was that scheming b**ch Jiune behind it?