Any GSD that 'turns aggressive' has done so because of how it was raised and/or trained (or rather, not trained). GSDs, as a breed, are very smart and very energetic dogs. These are not your lay about the house couch potatoes. They do not do well being left at home and ignored all day. Being so smart, and with a very strong working drive, they must have something to keep their mind busy. I have a German Shepherd whom I got because a friend had a single puppy left from a litter that nobody wanted, figured he'd be great for a running and bicycling partner. Didn't take long before he'd learned every basic command I could think of (sit, lay down, roll over, come, stay, heel, fetch) and I started looking up other things I could teach him, just to keep us busy. At just over seven months old, he already opens (and closes) doors, turns lights switches on and off, opens the fridge, can find any member of the house using the command "where's -insert name here-?", will pick up any item he knows a command to and bring it to you (ie. bring me: newspaper, ball, rope, shoe, magazine, etc), digs on command, and will even lift the toilet seat for you... I didn't teach that one, you can thank the roomie for that.
Long and short of it, if you have the time and energy, these are great dogs. However, if you don't, these guys can be monsters.
A sibling of my dog went to a home where they apparently thought they could just leave the dog alone inside all day, and never take it on walks and not train it anything. They returned him to the breeder after he ate the couch, chewed the legs off the chairs, chewed the bottom half of a door off to go into the back yard, and 'redecorated' their house in general. The breeders (who are friends of mine) found a new home for the puppy, a home with three kids, and the dog couldn't be better, no longer destroys things out of boredom, is super gentle with all the kids and is learning basic commands.
The only injury I have ever seen a German Shepherd cause to it's owner was one of the females from the breeders I got my dog from. On a walk one day with the husband of the house, a pit mix came out of a yard after them. He pulled the female GSD behind him and placed himself between the two dogs to try and stop the pit - who bit his arm. The female GSD jumped up and over the guy, using his shoulders as a push board to get to the pit bull, whom she bit and drug off of her owner. She left scratches on his shoulders from her claws as she jumped over him - he had to go to the hospital for stitches for his arm though, from where the pitbull had bit him.