Hello, I just came across this post and I hope I'm not too late to join the discussion.
Boon ok is a character intentionally created for the viewers to bestow their hate upon. Yes she is impulsive, jealous & somewhat selfish and tbh not the type of person who can make a good first impression but there's still a lot of depth to her character which you have already mentioned perfectly.
I would still like to add some more points about her:
Boon ok was never respected by anyone in the dorm (I agree Youngro was nice to her but she never really took a stand for her when her roommate bullied her)
She was always treated like a machine for being just a phone operator who was supposed to take calls for the students, and not get ahead of herself and behave like a normal girl.
What many people don't realise is that she was also of a similar age as the students in the dorm who can have dreams of her own - like studying, having friends and going on dates or various other things. But she was deprived of these facilities because of her family background and having to earn for her father's treatment doesn't help either. It's actually natural for her to feel this way towards them, especially when they all look down on her and call her names (like gizzard, wench, etc..)
Honestly speaking I don't see a problem with her when she was able to control them by keeping them at her gunpoint in the prayer room. Her action was nothing but a reaction to the behaviour of those students. Many viewers were angry with her for slapping and pointing a gun at Ko Hye Ja but did people really see nothing wrong with her behaviour towards Boon ok? She always looked down on her and called her names, often bullied her and even said bad things about her family. I was happy that Boon ok was finally able to take a stand for herself and get revenge. Besides that I genuinely want to ask - how many people will tolerate this kind of behaviour, especially the viewers who hate Boon ok for slapping that girl. I think Boon ok's action here is justified.
Another thing I want to point out was the infamous gun scene. People have been calling her out for betraying Young Ro but wasn't it her who kinda went behind Boon ok's back and decided to go out on a date with Suho after they met in that record shop? Ok I know it doesn't make sense to compare these two situations but if we look at it from Boon ok's POV - she already mentioned in the earlier episodes that Young Ro is the only person she can trust in the dorm and she even favoured her many times. Her behaviour towards her changed only after she felt betrayed cuz as u already mentioned before - their friendship meant nothing to Young Ro but it deeply affected Boon ok. And she just pointed the gun at her to provoke Suho, she wouldn't have actually shot her. Didn't Suho also point a gun at Youngro earlier and call her a human shield? Then does only Boon ok get hate for doing that? She was also using Youngro as a human shield to rescue the doctor and Gyeok chan - the only people she trusted at that moment.
Having said that, I lastly want to appreciate Kim Hye Yoon for portraying this kind of a character and justifying her role. She probably knew she would get a lot of hate from a certain fandom (I'm not going to name it cuz you probably know) but she went ahead and accepted this offer anyway. Hope to see her get a main role in her upcoming projects.