I have a 100 pound doberman. He's very very well trained. We taught him all the common commands, sit, down, stay, come. The fun ones, shake, little speak, big speak, high five, smile. Then we taught him the safety commands, he stops playing instantly if someone says 'Ow!", he will actually nibble food out of a hand with his lips, you can pull his ears, his tail, take his food, his treats and he doesn't blink an eye. He was 3 when I had my first baby and 5 with my second. He's very protective of them, always insists on going in the backyard if they are out there and has never once attacked another animal that has either been in his yard or that we've taken him to. He even plays with our 13 pound cat and runs away from him. That being said... he's big, scary looking when he growls and is very protective of my family. Which makes him ideal for protection.
We have guard dog signs up on my double gate and house. If someone I don't know comes to the door my dog comes with me to answer it. They can't say I didn't warn them when they walk right up to a sign by the door. But... I've been chased by dobermans and pit bulls. For these types of dogs running is the worst thing you can do. I stood against them and they backed off. A lone dog typically will. A pack, well those pretty much just have to be put down, especially once they become accustomed to enforcing their territorial dominance by violence. My family's mantra has always been, there is no room in this world for a dog that bites. I support that wholeheartedly.
That being said, most dogs have reasons for being aggressive. They were either abused and snap out of fear or were taught to be aggressive. Has anyone ever seen a puppy that was feral? Most likely not. There is a reason for that.