If you have good aligned PCs acting like Ted Bundy and murdering everyone left right and centre, you have bigger problems.
Id simply tell them 'No'. Then discuss what 'good aligned' means. If they tried again even after having that chat, I'd let them, change their alignment to 'Evil' and have them hunted down by a Champion, a War mage, a War Cleric, a few Knights, and enough Town guard to make it a Deadly x 5 fight (using all the divination magic at my disposal, plus Zone of Truth etc), and likely then having them publicly hung for murder. If they somehow won that battle or escaped, the next wave coming after them would be even more deadly (the local Lord posts a 25,000 gp reward for the PC in question, dead or alive) such as Assassins and the like, or powerful (good aligned) adventuring parties.