Tor is actually centralized in that ~five servers tell you about every other node to use, and they have the ability to tell you to use whatever nodes they want to...if at least three of them are malicious and colluding. The ability to do that without a massive botnet is of limited use though, and it would be noticed pretty quickly if the directory authority nodes suddenly started to say the Tor network consisted of an entirely different set of nodes than it did five minutes ago.