Eunha is Beatrice, oh well how to put it better, as Bea would die out, the playwright realized this late but she was looking for someone who would give Beatrice a soul ............. there it is where Eunha enters, at the beginning of the novel Euhna explains at the age of 9 a fortune teller told her that she would die young because her life thread was short (This is only put in a small scene in the manwha in chap 1 where the fortune teller only says " You will die young "), but this was because the playwright already had her in her sights to turn her into Bea's soul and in fact it was she who killed her, she was just waiting for the exact moment when Bea's soul could no longer put in In her body, Eunha's soul, hence, would process to erase Eunha's memories to insert what little was left of Bea (A little of herself, mostly memories and abilities)
This means two things, the first is that Eunha read her future, since the Beatrice in the novel that Eunha read was herself but turned into Bea, the second is that Bea herself is dead and alive at the same time, our Rae knows things related to religion and coincidentally Bea was a theology student before she died out.
Now, the original Raeliana, is effectively as you said, she wanted to be the protagonist, the reason is simple: she ended up hating her friend (both Beatrice before she died and our MC) for feeling inferior towards her, after Evans (witch of the previous chaps) told her that Francis would kill her, she began to visit her more and more but secretly just to know how to change her life as an extra, she did not ask Bea for help because of the hatred, that her parents have for her and that in a way she is not looking to save her life, but to change, hence why in chapter 97, when Vivian asks her why she wants MC dead, she only answers "Because I want to change my destiny".
The reality is that Evans offered to be Beatrice and that Euhna would come to take the body, she did not want at first until she heard the following:
"You can drink this at night and sleep. Then TA DAN!, You will become someone else! Are you excited, aren't you? If you do, you will become the wife of a rich, powerful and charming man, and there are so many people who will love your bright personality. You will be attractive and have a beautiful smile. You will live a happy life, receiving everyone's love until you die, "Evan said with a laugh.
"Because 'Beatrice' is supposed to be like that."
Currently, she is very frustrated and furious because her plan to be Bea and conquer the duke to have her fairy tale Happy ending went to hell when Raeliana / Euhna started doing things that captivated the high rankings and made Noah fall in love with him. her chance to finally have a man who loves her, since most of the nobles who almost committed to her when she was still in their body called her boring and as interesting as a stone, most did not want to be with her because of that. She thought Francis would be different, but she was wrong.
This is why she thinks she deserves to have Bea's life more than Bea herself. Obviously not right in the head.