FrozenNorth
Hero
It is clearly ethical to prevent future harm against others.I only know about the setup what's described here in this thread, not having read the adventure myself, but from that I would say that any option that does not involve the NPC answering for their crimes is not an ethical solution.
So peacefully buying them off to stop future murders and walk away scotts free (with the extra money in their pockets) is not an ethical solution.
Preventing harm against others by any means necessary is less ethically clear cut. Particularly if other options exist.
Punishing wrongdoers is also less ethically clear cut, particularly when you are not a police force or are otherwise empowered to act in such a capacity.
The PCs are not the police.The police finally confronting the drug cartel and being "here, just take this 50 thousand dollars and stop selling drugs in the future" is not how it's supposed to go down.
The PCs trying to apprehend and using all the necessary force to do so, is not being murder hobbos.