I can understand a moral opposition to killing the bullies, but would it be narratively inappropriate for that to be the final stage? If Yeon-jin really drove So-he to kill herself, is it a poor form of revenge to eventually do the same to her? Further, if Dong-eun were to feel satisfied by killing the bullies, wouldn't that technically be the best revenge (since it is her revenge)?
Now, I agree with you that flat-out murdering the bullies isn't particularly satisfying from a narrative standpoint (which is why it was never going to be step one, if death is even in the plan at all), and I'm not suggesting anything about the moral opposition to killing someone in cold blood. But I think there's a clear separation between what feels like a reasonable outcome (because this is a story) and what would be the most equivalency-appropriate balance for what the bullies got away with in the past.
Death is too quick, certainly--but that doesn't mean the bullies didn't forfeit their humanity by their past (and, conveniently, still mostly current) deeds and, as such, forfeit their rights to be treated humanely.
On the other hand, death is quick, which means that there's way less chance of innocent people wandering into the crossfire by accident like an elaborate scheme leaves room for.
The other thing to consider is that our protagonist is out for revenge, which is itself an immoral act. Which, to me, means we're looking at the guilty attacking the guilty in an isolated pocket of guilt, which--narratively--means everything's fair game.
...until the innocent start getting hurt, and then we're back to square one.