I absolutely 100% knew *exactly* who the hacker/culprit/killer was from the set-up. Reading lots of YA thrillers can sometimes be a let down because it can make you scrutinize small details that make "twists" feel unsurprising and disappointing. Let me clarify, however, that I WAS NOT let down by the "twist"--even though I knew WHO the culprit was probably going to be, there was MUCH more to it than that...you know, motive, execution, etc....And let me tell you something, I think I liked it better having this "idea" in my head.
This book accomplishes some pretty great things with it's character's--there is a very distinct feminist message that this author accomplishes by using these character's very distinct and different personalities, motives, identities, and desires and crafting them into complex characters that represent the way a lot of media, even social media portrays power in relationships AS WELL AS who is targeted or blamed when the relationship isn't really what it appears. There is so much internalized misogyny in the way the media looks at relationships, so it was really interesting to have this dive into the mindset of these female characters, who are all QUITE aware of how HORRIBLE the guy they are with behaves/treats them/treats others, but still, at least for the first good portion of the novel, are pit against EACH OTHER because of his bad actions. Even when the three main girls are having things slowly exposed to them, they would seemingly rather EXPOSE AND ATTACK EACH OTHER rather than go after the man that did this to them in the first place.
It isn't until the final chunk of the book that they each sort of confront that part of themselves that we see the actual pieces of the story unravel and the "twist" culprit, when revealed, seems to just give more evidence to the narrative that instead of "hating the girlfriend"(or the side-girl, even), the true villain in the story was the crappy guy who cared more about his "image" than he ever did about anyone but himself.

If you ever wanted someone to spin "Cell Block Tango" into an excellently crafted teen thriller, you're about to get your wish.

A poignant spin on the lengths people will go to for fame, and an unexpectedly raw portrayal of what being a "girl's girl" often becomes in culture... and what it means to truly become one.