We have two Schnauzers, Perry and Daisy. Daisy barks at everything but would not hurt a fly. Perry on the other hand we have to be very careful with, we got him at nine months old from a breeder and not sure what his first months were like. He is very protective of us, we have learned he will not be people, kid or dog friendly. Sometimes when friends come over we put a gate between the living room and he gets used to them being there or we have to put them out on a patio. We did take him for one on one lessons and it didn't do much for him, so we have concluded that we just have to be very careful where we take him, when someone comes over, and when he is out we always have a body haulter and leash on him.
At home with us he is a sweet angel. I think some dogs are prone to be that way, whether it be their background or just their personality.