The sad thing is that real people behave like this.
A lot of people are so self-centered that they cannot see things beyond their own world.
They might be *nice*, like how this FL started , but because they are unable to see the world from other people's perspective, or consider a world exists independent of hers. She's not intentionally "selfish", not really, because to be selfish, you need to know where the boundaries are. But because she is willingly ignorant, she ends up tramping on other people's territory easily. That's why this type of people/characters are often the most frustrating. Because they hide under the excuse of "I don't know", even though they had the responsibility to know.
The demon lord in this case is actually selfish. He knows exactly where the boundaries are, but choose to ignore it because he puts his life over everyone else's. He willingly chooses to trample over other people for personal gain. He's still unlikeable, but he's not as frustrating as Feng Wushuang because of this.
The only difference between now and before, for Feng Wushuang, is that she is starting to be aware (although not by much), and willingly sacrifices other people now. She is starting to become both selfish on top of being self-centered.