Oikura knew Araragi’s parents were cops because they were the ones who got her out of her abusive home and had her live with them. Araragi can’t remember on his own, but that’s not entirely why Oikura despises him. As we learn during one of the more powerful sustained monologues in the Monogatari franchise, and a chance for Inoue Marina to remind us just how good she is when she sinks her teeth into a role.
As hostile as she is to both Araragi and Hanekawa (throwing tea at the former, which the latter catches with her cat-like reflexes), she still seems to get a lot off her chest and be better for it. She also comes off like never before like a deeply wounded individual; a lost soul who has given up hope.
It’s already the end for her; after all the punishment she’s endured in her still short life—physical and emotional—she believes she’s too frail for happiness, so she despises it along with herself and everything else in the world.
That punishment includes having to watch Araragi’s perfect family seem to “show off” in front of her. She’d glare at them in resentment, or for not knowing how different they regard “normal family life”; in other words, how much they take love for granted. Oikura was never given love. Her parents divorced, her mother became reclusive and never left her room, and Oikura had to take care of her, until one day she was just…gone.
After all that, Araragi forgetting all about her and giving her nothing in return for what she gave to him throughout their encounters, reveals itself as simply the tip of a very nasty, despairing iceberg. Inoue mixes dread and malice with tones of black humor and feigned happiness in Oikura’s delivery, heightening her aura of imbalance; a spinning top about to fall off a table.
She made Araragi a villain despite his relatively small contribution to her wholesale suffering because she needed to blame and despise someone other than her parents (which neither she chose nor who chose her) or herself just to keep going on. Whenever she got near the happiness Araragi seems to ooze, it felt either too bright or heavy for her frail, scarred self to survive.
Happiness, she believes, will kill her just as efficiently as the emotions on the other end of the spectrum. So she’s settled for something a little more moderate on that scale, and it’s slowly dissolving her heart. Araragi tell her happiness can’t do that, and there are many kinds that would work for her. But Oikura lacks the ability to access them.
What she needs now, more than anything else, is to continue being heard, and being in the presence of others. When she kicks them out, Hanekawa says both she and Araragi will keep coming back, because “troubling those we care about is how we do things.” It’s pushy, but it’s also something Oikura needs to hear: someone cares about her; is fond of her; and she’s several decades too early to be talking about endings.