She isn't a villain, just the current antagonist. The reason they are making her monolog so much is so we can be empathetic to her later on (I probably won't still)
She's clearly not the villain, they HAVE personalities It was a reference to the fact that in many sources of media, the villain will have an especially long monologue (which the irony is the fact that she isn't, she's a small supporting character and therefore should not have so many lines). The last page was the only one that included a flashback/back story. She literally spent 4 to 5 pages just insulting the MC/talking obnoxiously to herself and by page like 9/10 he finally responded to her.
The reason they have her talking so much isn't to make the readers feel empathetic but it's to highlight her inferiority complex and extremely low self-esteem. She's a jerk to the MC because she's projecting her insecurities onto him because she does not have an established/stable spot within the household and the only one with a more unstable spot is him (as he's shunned by everyone). But the MC has been able to make headway and slowly accumulate respect unlike her who is at a standstill. So the person who is supposed to be permanently at the bottom of the household and who should never been able to pass her is now somehow threatening her position. She thinks if she can beat the MC and squash those 'below her' it will help solidify her spot and confirm her worth/abilities, because if she isn't talented her value is therefore zero and she will be considered useless (and therefore can't complete her goals).
Also, the whole tragic backstory to redeem the character's crappy personality is BS because not only is some random quick minor flashback poor writing, we all know she's only getting enough of redemption to lead to her being a shitty romance choice who follows him around in the future (probably as a tsundere because it's the easiest cop-out). I'm praying there won't be romance, but authors always manage to sneak them in
embarrassing.... just plain embarrassing dude
(I skipped a good portion of her dialogue this chapter because seriously? even villains don't monologue that long the fact that she had a 1 sided conversation that long was just sad...like girl.... he's not listening. you look stupid now and you are going to look worse later )