the MC is burned to death alive leaving behind a child under her brother's care

Although I did not read it because the translations stopped but I read spoilers from the forum about the ending and it was exaggeratedly dark. I do not know if the author has reservations toward incest or something but the MC SHOULD HAVE ENDED WITH HER BROTHER THE ONE SHE LOVED IN THE FIRST PLACE, instead, her vessel broke and she could not use magic anymore and she ran with Atlas from the country and her brother became the king or something and they had a daughter, adding to the dark element of the story she died a horrible death smiling (burned) in front of her daughter because the magicians were being hunted down by the people because the people saw them as inhuman beings. This kind of ending because either the author against incest or he wanted the end to be heartbreaking, but whatever the reason the ending seems to be forced to the novel, so I am disappointed from the novel because of the last volume and it affected my whole experience by just remembering it.

She was born in an era in which mages are dying out. The world was moving in a direction that discards magic and turn to technological development. You could say the MC was the last prodigy of magic that represents the old time that worship magic. Therefore, she is targeted by forces inside and outside the Kingdom. So far, our MC is well known as the princess of inferno on the peak of magic; hence she can single-handely ensure mages' social status. As you can see, her mere existence is intolerable to the new world that is about be born. If she falls, which is expected by her many enemies, then the rest of the magician society will collapse, including her brother that is yet to make a name for himself. Therefore, our MC has to stay strong and kill many until her brother can grow into the destined hero.

Origa has a daughter with Atlas. They changed names and appearance and live somewhere else. Her brother, Gil, eventually built a kingdom that uses a barrier to separate mages' society from the common society. You could say the story has a happy ending. HOWEVER, the epilogue tells from their daughter's pov that Origa, the MC, is burned to death smiling in a witch hunt ten years later.

If she stopped to think maybe, she could have found an easier and more effective ways to save the beloved protagonist without screwing herself over. Although it explains the want of her destruction, I still don't get it at all. It seriously makes no sense to me because it just sounds like she's doing it because that's what her role is supposed to be. But logically speaking no one wants to die and if she questioned the good of playing the villain role in the off chance it helps her brother develop, then why couldn't she stop to think to play a good person?