That's one of the reasons I mentioned both commerciality and length. Someone who pays for what's marketed as a fluffy otome game has an expectation of happy love-love with at least one gorgeous guy. Even in a free game, the longer they've played and the more they've become invested in what they expect, the more cries of 'unfair!' if you pull the rug out from under them.
But to run with Taleweaver's idea, I think it could work out fine if:
The number of bad guys is limited (since the shock will only happen once, having a lot of them would be wasted time, people wouldn't want to play a bunch of paths knowing what would come at the end). Probably no more than three.
There are hints about the bad guys being bad or having been bad in the past. A brief glimpse of them being rude to a less pretty girl or joking about nasty things they did as kids, possibly in a way that will be intentionally hard to understand on first glance.
Ongoing hints that the narrator is intrigued by the guys, but not in love with. Done right, this motivates the player to keep pushing because they expect the protagonist will turn romantic eventually if they just spend enough time with the boy.
With those bits in place, the reaction to the murder is more likely to be "Oh! THAT'S what was going on!" for at least some people.
Optional: finishing one or all of the murder routes unlocks a new route/epilogue where you can date a fellow bullying victim, who might even have been helping you behind the scenes all along. And this romance is a real romance. A romance between crazy vengeful murderers, perhaps, but the player would finally get the romance element they came for.
With a setup like that, some players would still be very angry, but I think a number of them would appreciate the twist. Especially if the bonus route were included.