There are aggressive Yorkies and there are non aggressive pit bulls. It is all down to genetics and environmental upbringing. Each case needs to be judged individually. But, at the end of the day, it is not the dog's fault. Human interference is what causes problems from irresponsible ownership and mismanagement. And the way some people treat dogs for their own benefit rather from the benefit of the dog.
Anyone that owns any type of dog should do their very best to ensure the happiness of that dog and ensure it does not cause any unhappiness to any other dog or dog owner.