Every great community has value and values. Value, what are members getting out of being a part of the community. Values, a strict code of conduct – explicit and/or implicit, that every member follows to uphold the quality of the community.
I think that cycle sometimes happens, but I think it’s more common for a community not to be ruined by sociopaths, but rather too many mops, whose mere presence taints the community and the brand for everyone else.