Awww man! A murder suicide attempt, pretty tame, and it's on the extras? Oh well
I think I know what's going on, from a shrink-y point of view.
I've watched, and you probably have too, many movies and shows with a gay guy who has married a woman and is completely repressed about his true sexuality—because he has deep internalized homophobic prejudices.
Then he meets a comely boy and falls for him and they start a thing. And it's seriously hot. And you think okay, now this guy will finally come to terms with his real self, out himself to his wife, get a divorce, fix his life, all that stuff... right?
Well no. Next thing you know, the guy visits the pretty boy, goes bananas, throws the poor boy on the ground and starts strangling him while he yells: "It's all your fault! You turned me! You goddamn fag! I'ma kill you and then kill meself!"
I think Yuni, at that moment, in that scene, goes thru a similar mental process. Feeling a sudden disgust for what she has become (a cheater), putting the blame for everything on Fuuko, and then thinking about murder and suicide—why, it's a classic example of self-hate mental gymnastics, completely ignoring that her toxic relationship with Nanase Natsume is the source of all the problems here!
It's good that rationality prevails in the end and she stops herself. Otoh it's too bad that in ch20, when Nanase asks her to return to their previous status quo and pretend nothing has happened, Yuni says okay. That was the irrational choice, the wrong course of action, and won't lead to anything good.