I've never met a mean rottie. However, one of my uncle's rotties had rage syndrome and had to be put down. I've only ever heard of rage syndrome in springer spaniels, but either way it makes the dog too dangerous, especially for the owner themself. But this is extremely rare and can happen in any breed.
Rotties aren't meant to be aggressive toward humans. They are meant to work directly with their humans. They even have a long history with children. Being used as a cart-pulling dog on farms often meant pulling the little kids around the yard in the cart as well.
They do have a bad reputation though. It can be hard sometimes to own a dog and have people cross the street when they see you, or tell you to put the dog down just because of its breed. The neighbor of that same uncle once shot one of his rotties because it was roaming too close to his property. We're talking farms with acreage. He couldn't let the other dogs out of his sight because the neighbor's had this idea in their mind that all rotties were bad, and they weren't giving the dogs a chance.