Alexander the Great or Napoleon have been limited in how much damage they can do, because they're human too. They don't live for hundreds of years, they can't copy their brains to new hardware, and they can't modify their own minds to remove that pesky conscience when it slows them down. Perhaps most importantly, Napoleon's happiness, power, and continued survival depends on having a civilization of surviving humans working under his command, whereas that might only temporarily be true for AI.
At best, history serves as a counterexample to the idea that if an AI goes bad the attendants would just unplug it, seeing how often it is that dictators don't get stabbed by their aides as soon as they start causing mass deaths, instead often receiving broad popular support as the world burns.