Dio's pretty left wing...
"The world is full of kings and queens who'll blind your eyes and steal your dreams/They'll tell you black is really white, the moon is just the sun at night, and when you walk through golden halls you'll get to keep the gold that falls" (Heaven and Hell)
That's a comment on capitalism and gentrification... he's saying that one of the lies they'll tell you is that if you are surrounded by wealth, that it will in turn influence and inspire you to rise to that level (you get to keep the gold that falls from the wealthy). Also, Sepultura, Testament, D.R.I. and Anthrax are all left-wing. Black Sabbath too... Motorhead is anti-authoritarian... Theres this german thrash band named Rumble Militia that are openly communist. Also the supergroup Brujeria are also openly communist and sympathetic to the Zapatistas. Dying Fetus is radical left... so is Misery Index. Brutal Truth. Terrorizer. Benumb. Cephalic Carnage. Assuck. Napalm Death. Extreme Noise Terror. Crust bands like Amebix, Disrupt, Anti-Cimex, and GISM: I don't see why any metalhead wouldn't like those...
I don't know about any pagan metal bands that are left wing though... I mean, if you think about it, going back to your pagan roots for musical inspiration is pretty nationalist... I feel you man because I really like a lot of folk black metal and viking metal and they really do tend to be right wing... Maybe you should check out Hedningarna, they're a swedish folk rock band (Finntroll actually covered one of their songs)... they're just a bunch of harmless hippies...