I've heard this nonsense my entire life from people who just don't know better. My father trained dogs for many years, first for the K-9 Corps, then privately. He taught us how to handle dogs when we were very young, and at 60, the past two years are the first time in my life that I've only had one dog in the house.
Hard to be a better dog owner than me. I won't punch down the list of many of the extraordinary things I've done for my dogs, but I will tell you about one dog I had.
I got GG as a puppy of about 6 weeks. She was some type of mixed breed, but there was some Rottweiller in there and possibly some GS. At 80#, she was a big, strong girl. No dog that I've ever owned has ever been struck by a human or mistreated in any way, and I've taken in dogs that have clearly been abused. GG was not one of them. She did, however, have a screw loose. She tried to bite a person the first time at ~4 months old, and not the 'I'm a 4 month old stubborn dog' kind of biting, but the going after someone until she had to be restrained type of attack. This behavior would continue until I finally lost the old girl at 11. She was one of my favorite dogs and was fiercely loyal to me and tolerated other family members well, but she had to be kept away from other humans and all other dogs except for one for her last 8 years, and when a family member made a mistake and didn't secure her properly in the area of the home where she was kept, she killed a 60# Husky. She was 9 and incredibly lazy when that happened, but not too lazy to take out Bailey.
I spent many, many hours working with GG on her behavioral issues. She was very well trained re: obedience and was quite bright, but again, loose screw. I could put her in a harness along with the other big dogs and have them pull me on Land Rollers along a 3 mile loop in an effort to burn off her excess energy and aggression, and that was effective to a degree, but at the end of the day, she was what most would consider a 'bad dog'.
You're free to keep thinking that there are no 'bad dogs', but like people, there are some animals that just can't be socialized regardless of how hard one tries. Are they really 'bad dogs'? WIthin the context of being unable to be around other people or animals without biting, then yes, there are plenty of 'bad dogs'. Was GG a 'bad dog'? Not to me, but if she ever got loose, there was trouble.