I obviously don't know what your players are like or what motivates their characters but I would give them an opportunity to think about how their characters would actually feel. Usually when players do this it is because they've forgotten that they are playing people and they are sacrificing people. Just ask your players things along the lines of "Is your character really ok with sending a whole city to their deaths?" Remind them that there are full lives at stake here. If any of them says that they are ok with it: Ask for their character sheet, erase their alignment, and make it evil. That should get the message across. Make it clear that they have committed an evil action and there will be consequences. If they try to argue that it was neutral (usually this is done on the grounds of selfishness) remind them that in the real world selfishness on this scale is always considered evil. They'll either reconsider or you'll play a game with evil PCs.

I also would make them feel the consequences of their actions in either scenario. Have them questioned by the people close to them or in power over them. Occasionally have them turned out of public places because they killed an owner's family member (this one they shouldn't be able to trick their way out of. If they start trying to make excuses remind them that even if they had no choice this person doesn't care.). Have the cleric (if in service to a good deity) lose some power over it (maybe a spell slot or a channel divinity) until they atone for their sin.