I 100% agree that we as a mature society, should be there and be prepared to tell them why they are wrong. But I think you are overlooking the possibility that they shrug and start killing. We should be prepared for that too.
If it comes to judging „what a truly tolerant person should do“ you make an absolute normative, hence moral claim.
This strikes me as rather inconsistent and therefore unconvincing.
On being 'confronted' with the question about legalizing genocide. Well, if there ARE people with such ideas, it's not like making the ideas illegal will stop such people from having them. So open discussion about it and democratic voting will have just 2 outcomes: nazis will be either reduced to nothing and their political base will dissolve, or they'll get to power and a genocide will happen. However, I believe that a society that will openly vote for genocide is beyond saving, whether you criminalize some ideas or not.