I agree, that can happen. It can happen in any dog, but these are generally long-standing issues with specific dogs. Growing up I had a lab that was great around our family (and our other dog), but extremely aggressive to anyone else. My parents ended up having to put it down because anytime it saw someone that wasn't us it would snarl and bark and try its hardest to attack them. We lived outside of town out in the country so it wasn't a huge deal but still pretty bad.
I guess my point is that yes, dogs can have issues like that. Some are just plain aggressive and there's little the owners can do. However, most are not like that and it's generally up to the owners to raise the dogs properly and if they know that their dogs are aggressive, they need to take steps to address that or get rid of them. I have no issue with that.
But when someone goes around spouting that an entire breed is dangerous just for being that breed, that is ridiculous and that person needs to be checked out mentally. Applying that thought process to humans would make them a horrible shitty racist and in general, that's how I see it.
Dogs have the potential to be aggressive regardless of breed but it is ultimately up to the owners to address those issues.