These are all very good points, chrisreader. I don’t think James was as completely redeemed in the book as you do, although we have evidence that he may have turned out to be a better man. I definitely don’t think Snape is either totally redeemed, or totally heroic. I like that about the character because it helps us see that people are not all good/all evil. It’s great to have the Beatrix or Valdemort characters to defeat because they are pure evil and that makes us fell good. Snape is not pure evil. He is so much more complex. James Potter was not pure good, he had things to overcome as well. These are much more like the people we’re likely to meet in life.
The Neo-Nazi comparison is a good one, and yes, I think it helps us to see that even those who espouse dangerous ideas are not caricatures, but real people. I’ve read some stories behind some of the people in these groups and some of them have definitely been sympathetic characters, misguided but with understandable anger and hopelessness. And the fact that many people quit the groups and even the Republican party after Jan 6th shows that when faced with hard truths, some people can step back and question what they thought they knew.