Everything is rate-limited on SE sites. And by everything I mean also moderator actions like destroying users. So a user running amok will run into some of those limits, slowing him down.
High reputation users cannot do anything that cannot be easily reversed, so they can't really cause lasting damage. Just flag for a moderator when you see a user vandalizing the site and he'll be quickly suspended.
Diamond moderators can perform actions that are hard or impossible to reverse, stopping them and reverting the damage is something the SE devs have to do. As this hasn't happened so far there is no realistic estimate how much damage a rogue moderator could cause. There are some restrictions on hard-to-reverse actions on users that would stop a moderator from certain high-damage actions.