When a paladin or priest says they are bringing the holy fires purging out evil in witch burnings, or whatever, odds are it is regular fire and they apply it prolifically on anything they themselves label evil. And then claim the proof it was evil is that it worked.
So in that case, what you - as a village er/chief - should do, is require that they perform the exact same evil-purifying attack/ritual on themselves and their subordinates to prove that it truly only affects evil, before letting them use it on your peers.