Have you considered that people support Billy/Harringrove not in spite of Billy's prejudices, but because they want him to grow as a human being beyond them? I'm not a Harringrove shipper but it's obvious that Billy is a product of a highly damaging upbringing. He's lost his mother to a divorce (presumably) which forced him to grow up with an abusive and hateful father. Of course he has issues! People support him because they want him to rise above these demons and become a better human.
Sure they can give Billy a redemption arc but in order to become a better person he needs to actually want to change. The reasons you described for supporting Billy could be applied to his own father, or any other abuse piece of trash, anyone can have the potential to change but it only counts if the person themselves actually realise that potential. A lot of people grow up in poor home environments including other characters in the show (El, Jonathan, Will, Max) but none of them have done anything even close to Billy’s level. Furthermore all of them have shown a willingness to change and actively try to be better people within canon. There has been no indication of any willingness to change from Billy and until I see that I’m going to reasonably assume that he’ll continue his racist and abusive behaviour.