The thought that only killer dogs are suited for these activities is wrong and can be extremely dangerous. Problems arise when people want to make the "baddest dog on the block" or are looking for the most aggressive dog to teach to attack. Or, when people enter the activities half-heartedly and poorly train the dogs. This is when we end up with dangerous dogs.
Those who are dedicated to the sports of Schutzhund and Personal Protection are just another branch on the tree of dedicated dog lovers who enjoy the bond training creates and the feeling of working closely with their dogs.