I do actually welcome controversial lyrics and imagery in metal. I see it as sort of a canary in the coal mine for freedom of speech. With lyrics that are racist, misogynistic, militant, or even pedophilic, it's hard to tell how these bands straddle the line as far as actually advocating the ideas in their lyrics vs. wanting to evoke negative emotions and shine a light on the ugliness of humanity. The band Cynic summed up my view of extreme metal nicely with one of their demos titled "Reflections of a Dying World", and so in that sense I think even the most heinous lyrics have their place in metal, because anything else would be ignoring who we really are collectively as humans. I also find it a bit odd that the metal scene gets particularly worked up about racism, whereas murder, rape, torture, etc. are more-or-less par for the course in extreme metal nowadays.
What does ones political political alignment has to do with music?
That said, I've been on stage in Germany when Neo-Nazis tried to enter the venue to attend the show. The looks of legitimate distress on the attendees faces are something I won't forget. Personally, I find it really hard to separate art from artist and artist from political alignment.
There's no room for Nazis in metal.