I'd bet most of them were raised that way and just don't have enough outside experience. I have two data points:

1. I worked with an otherwise very bright guy who thought I was insane for wanting to visit Western Europe, because they'd slit my throat for being American. And this was 10 years ago! He's lived a sheltered life but doesn't realize it, and get too much news from one source.

2. My grandmother, one of the sweetest people ever, warned me about "negroes" and staying away from them. She was just a product of her upbringing. Over the years (and after moving out of her small town), she very much changed.

Not that many people really want to be evil. Some do, but not most. I find it's usually ignorance. Talking nicely with them is what changes them. I personally hold a belief, and someone from the opposite side of the issue has engaged with me. I ponder it quite a bit, and see their side more humanely than previously.

Are you conflating "alt right" with "white nationalist"? If so you're either falling victim to, or participating in, a propaganda campaign designed to discredit all opposition by conflating it with its least savoury elements. You can't just go around calling everyone you don't like a "white nationalist", that's a term with a specific meaning. It's like saying "all leftists are communists".