See the weird thing is, Rinon looks like the one in the wrong here for being upset by Furutsuji's actions, but that's just because she has a different relationship with her than either of them.
She's been scared of Furutsuji since chapter one. To the other two, Furutsuji is just a nice girl, she's no threat at all. But she is a genuinely unhinged person completely obsessed with Rinon. Rinon is (not fully consciously) starting to realize the threat that Furutsuji represents to her. That's why she was so alarmed to see the extreme violence she was willing to use against the creep. To the other two, it seems weird, 'cause fuck that guy and he got what was coming to him, but they're not the ones she has in her sights. They don't realize how scary she is to Rinon because she just isn't a threat to them in the same way she is to Rinon.