Training doesn't change temperament. If a dog is "mean", it will be mean regardless of it's training. If it isn't mean, good training certainly isn't going to make it mean.
Sure there have been SchH dogs that have harmed people. They're dogs. It happens sometimes as there are some unstable dogs that exist in the world. Though truthfully there are probably far fewer amongst SchH dogs than amongst the general dog population because SchH weeds out instability of temperament and also puts a level of obedience and control on the dog that far exceeds that of most dogs running around. Before a dog can even trial for a SchH title it has to pass a very comprehensive obedience and temperament test called the BH.
If dogs fail the owner trains more and enters another trial and tries again. It happens all the time just as in any sport. Sometimes you pass, sometimes you fail. No big deal.
If the dog doesn't want to do it (or more likely doesn't have the right temperament to do it) then most people either find another activity for that dog or another dog for SchH. A dog who has suitable temperament tends to love SchH training, just as agility dogs love agility and flyball dogs love flyball and herders love herding. Sure, there are some people who try to get their dog to do something it isn't well suited for, but again that happens everywhere and is pretty uncommon. Training for SchH is extremely expensive and time consuming. Most people aren't going to put all that effort into working with a dog who doesn't enjoy it or isn't cut out for it.