TIL this sort of derailing of a conversation is called 'concern trolling':

Concern trolling is a manipulative tactic where an individual pretends to be genuinely concerned about a particular issue or cause, but their real intention is to derail the conversation and discredit the opposing viewpoint. In the example you provided, person B is attempting to associate the contentious idea with Nazi thought in order to make anyone who opposes or questions the idea appear sympathetic to Nazis.