Point out that the witch finders thought that they were right too. The slave owners of the deep south in the US, Pol Pot's followers - they were all absolutely convinced they were right and so justified in what they were doing.
Worth maybe explaining about the Nuremberg trials and the huge efforts gone to by the allies to treat war crimes and war criminals absolutely by the book, with defense lawyers, with every single care to do it objectively as would be done for any allied citizen, because there had to be a commitment to treating every person in an ethical way regardless of who they were, what they believed, what they had done, whether or not you were on their side or agreed with them. That's the contract that protects everyone equally. Because once you get into 'it's ok to treat THAT person like that because they're wrong and not in my gang' you're back into the dark ages, and all you ever have is the hope that you manage to stay in the currently fashionable group.