Are you thinking of The Apple? Or that weird society in Spock's Brain?I will join the others in saying that Humanity's the one that scares me. Not so much because of what we're capable of, when we get to warring or fear-mongering, but what we do, also, when we have the very best intentions in mind ...
Most notably in TOS, but throughout the franchise, really, there have been civilisations viewed by the Good Ship Enterprise, or whatever, as being "stagnant" or "inHuman" and the STARFLEET people will just pull the plug on whatever's controlling it. Most often, it's a computer. Even if that computer's KNOWN to be in control by that particular culture and they take great pains to keep it well-maintained and fully functional, so what? Kirk or Picard or somebody's put off by it and they put a stop to it, then and there. Usually leaving the poor bastards to pick up the peices, while the ENTERPRISE warps out of the system, so it doesn't have to monitor the fallout and reconsider what they've done. Again, this is done with GOOD in mind! At least you expect Klingons and Cardassians and the Borg to be pricks. It's not surprising that they invariably are. But Humanity's supposed to be the best that a species can ever hope to measure up to ...
And "Return of the Archons" and "For the World is Hollow" and (stretching a point) "A Taste of Armageddon" . . . .