Well, their cause is not bad, just even. But it is not justified by the way they act. I say bad, they have since long abandoned their original belief to retake the Reach. Altough that may still be a primary goal, i think it is more about revange and hate against the Nords. Would they be content if they get the reach? I think not, murders against the nords would continue.
Even if the Nords are the ones who drove then to be bad, they are still themselves bad and if they do not prove otherwise I say they are bad. Seeing as they attack all who is not forsworn and not only nords as well.