There's a shit load of black metal that isn't nsbm or about satanic Vikings in winter, I'll start with that. Rabm exists as does Christian black metal and some of it is actually good.

I've seen people suggest the satanism was originally a shtick for the whole counter culture part, think I've seen interviews saying they used it as part of protest to what they saw as historic loss of culture due to Christianity coming to Norway. For some bands, it built an evil persona they could run with and added to the whole atmosphere and aesthetic. I would never call emperor or immortal satanic bands, even darkthrone, more antithesist than anything, and yet all these groups are historically relevant to the genre. Usbm is usually more about nature and emotion than Satan in my experience (there are so many fucking bands though so...) black metal has changed so much since the proto days with batlord or 1st wave Norwegian, there's fusion with a bunch of other musical types that you can probably find a group you'd like.

There are neo-Nazis in every extreme sub genre, comes with the counter culture/outsider/etc, Nazi punk is a thing, guaranteed if you look hard enough you could probably find Nazi or Nazi linked art pop. Some artists are shitty people, Chris Brown is an example I can think of for unrelated genres, I feel it's totally possible to dissociate the artist from their work. I listen to Burzum knowing full well what he's into these days, mind you nothing post incarceration but still. I would argue that like with satanism some groups use the nsbm themes as a shtick for shock value. Internet comment sections are also shit.

Shock value exists in other genres, thrash for example. Tom Araya is Catholic, all of slayer's anti religious and satanic imagery was meant solely to shock and "scare people". It's nothing new, it's just pushed to the edge.