I think this is the direction Igarashi always intended to take it. While this scene was definitely a dramatic escalation, it's not entirely out of nowhere. The art's always had this slightly unsettling atmosphere to it, and looking back this chapter is making a lot of callbacks Kasumi and Haruyo interactions. Kasumi exploding with rage when Haruyo ghosts her parallels Mizuki's reaction to Sayori telling her she's not moving in; and Kasumi telling Haruyo she wanted to punch her afterwards and says that she wanted her verbal abuse to hurt is sort of analogous to the cutting; the panel where Mizuki says she'll call the police and won't see Sayori anymore is 1:1 with the panel of Haruyo saying she won't touch Kasumi anymore; Sayori and Mizuki's expressions when Sayori says "then I can forgive you" is the exact same pair of expressions Haruyo and Kasumi have when Haruyo touches Kasumi's lips.
Not to mention there seems to be a recurring motif with Kasumi and hanging. (Bottom panel of ch3p21 where the window framing evokes Kasumi hanging; the light fixture shape and placement at top center panel of ch7p13, which are absent in the reverse shot; bottom panel of ch8p22 where she looks like she's hanging from the light fixture).
There's been some pretty ample foreshadowing that this story was going to go some pretty dark places from the start. This chapter doesn't feel so much like a tonal change as much as it does shifting gears.
I think there's a lot of somewhat dark parallels between Mizuki and Kasumi, between their somewhat listless lack of a place to exist prior to the Class S role play, and their willingness to sacrifice for their partner who's now given them some sort of meaning. The biggest divergence is we've seen Mizuki actively hurt her partner and lash out at them, but I get the feeling that Kasumi is far more likely to turn that inward and attempt to hurt herself.
I like to hope that when the time comes, Haruyo will be up to the task, but I get the feeling Kasumi is going to tread in some very dark places as the shadow of her friend's suicide looms over the story. For all the darkness, I do think that so far Haruyo's been shown as a beam of light innocently breaking away the darker elements, and I hope she's able to lift Kasumi out of whatever place she finds herself.