|Just because someone trains their dog to be aggressive doesn't mean all of them will be.
I have to point out that is a bit of a myth. The American Pit Bull TERRIER is just that, a TERRIER. Whether you raise them properly or not when they reach a certain age, between 2-3 they sometimes decide they do not like other dogs. It isn't bad breeding, bad training, anything, it is that they are terriers. Terriers are bossy, some just more than others. It's more common in terriers but really any dog, once they hit 2-3 years if the dog is going to become dog aggressive it will regardless of breed, breeding, training, etc.
I understand people don't like them because they are large enough to take down a person. I have never heard of a chihuahua killing anyone. However look at the caliber of people owning the dogs that attack people. Most of the time they are people that shouldn't own ANY breed of dog, no less a powerful pit bull. They are people who own them for the "status" of owning one. The pit bulls that are pets and family members turn out to be great dogs, however if you join a pit bull forum you'll hear a lot of tales of people who own multiple bullies and have to crate & rotate to keep the house sane.
I have a terrier mix who liked other dogs until she was 2 years old, after that she got picky. She's never been super aggressive but she will put a dog in its place if it jumps all over her. My friend has a pit/lab [knew the parents] she loved other dogs until she was 3 years old, now she is VERY selective of who she hangs out with and will get aggressive if the dog she is meeting doesn't behave how she wants it to. I also think it's a female thing, I think males tend to be more chill.
In any case, I LOVE Pit Bulls and Am Staffs, I also love Bull Terriers. This always happens with popular breeds, it was German Shepherds in the 70's, Rottweilers in the 90's, and now it's Pits. I can tell you from working in a clinic 98% of the German Shepherds we see are neurotic and horrible and would take your face off no problem. 99% of Pit Bulls we see are friendly and easy to work with. The Pit Bulls we have labeled as 'caution' dogs are dogs that you know are owned by drug dealer type people, or people who own them for "status" and they only come in once every 3 years for rabies. The problem with these dogs is the wrong people own them and don't know how to properly control them. They don't know how to help a dog aggressive dog react better, they don't know how to socialize a dog properly, and honestly a lot of people just don't care. They are high energy dogs, again, they are terriers!