I guess it's fair to consider the good he did along with the bad, but these theories that he was just working as an informant seem extremely far-fetched.

First, the KKK would have been aware of what he was doing in the community, so one would imagine he would be highly scrutinized if anyone had any reason to suspect he was working for the law. Second, I would think the FBI would step up and at least tell his wife that he wasn't actually an avowed racist but working for them. They would have no interest in hiding the nature of his work for them today.

Judging by all the evidence, there is no reason not to believe he was a member of the Clan. And whatever good works he may have done, that organization's record speaks for itself. I don't think there's much moral gray ground here.