Reachmen are an entire race, Forsworn are just a fraction of that. The Reachmen who still live in Markarth stay in the Warrens which is a slum. The Forsworn fight beacuse they had their land taken, their family and friends killed, their religion and culture banned and their king enslaved and forced to do the bidding of the Nords. Also worshipping daedra doesn't make you evil, if it did then Dunmer, Khajiit, Orcs, Nightingales and even the Septims would all be evil.