I think it is both , yes some dogs are renown for being aggressive and should not be pets for people who do not treat/handle them correctly.

In saying that i grew up with rottweilers, 2 pures and a cross , and they didn't have a mean bone in their body my younger brothers and cousin use to see how many rocks they could put in the large males mouth at a time before he spat them out . The biggest problem we had with them is they would follow us to school .

but i guess when you are riding you have to judge a book by its cover as you don't have time to ask it 20 questions. Mace them all and ride like the wind