I wont go into details because it will take forever, but soomin is a narcissist. you cannot help or heal a narcissist. I think the last scene in prison means that even after hitting rock bottom, she still lies and still wants to destroy jiwon. she lost everything, she tried to kill jiwon, and still, she says that she is her only and best friend.
she is not willing to learn from her mistakes. she doesn't even think she made a mistake. and she never genuinely ask for help. and that's very consistent with narcissism. narcissists always think that they are right and all the world is wrong. they don't want to change, that's why it is quite impossible to help/heal them.
but,
I agree with you, this never occur in jiwon's mind, and it would have been a good idea to explore this side of her and of soomin's personality.
lastly, becoming a narcissist isn't something that happens to you. it's a choice. you choose constantly to hurt people in order to save yourself. everybody face hard time in life. not everybody becomes a narcissist. you become a narcissist when you choose to blame everything except yourself for the things that happens to you.
this level of personality, everybody have it, around 5 yo of age. but then, normal people grow up. narcissists choose to stay immature and to let other people coop with their non sens. and at this point, enablers enter in the picture: it's BECAUSE narcissists find enablers that they grow worse and worse in time. minwah because of his mother, soomin because of jiwon.
the only way you can help a narcissist is to let him/her down. and sure, they'll throw a hell of a tantrum....