Case in point my mothers springer spaniel, not known for being a "dangerous" breed, he was the perfect family pet really, he really wouldn't hurt a fly as the saying goes... even if another dog went for him he didn't react at all...
...except for cats. If he saw a cat he would go for it, and I mean if the cat couldn't escape and I wasn't holding his lead I've no doubt if he caught it it would be curtains for the cat. He wanted them dead, no two ways about it. Every dog has that instinct, once that switch is flipped, and if its a human that flips it, then lord help them. And though he was only a springer I was amazed at how strong he was he could knock you off your feet if he had a mind too. Only 9 or 10 stone, but it was pure muscle.