Karl Marx once said that the world would be better off if all Jews were eliminated. There is no reason why a Jew can’t be a Jew hater. But those ones tend to be the non-religious type, what the Bible would say are not really Jews.
Technically, they cannot be treated as a Jew. But if they repent, they do not need any kind of conversion. The just need to behave as a Jew and the past is overlooked.