Some dogs have more aggressive traits than others that goes without saying
However no dog is born vicious, it is completely (or near enough genetics can play a part) down to the up bringing of the dog. There are multiple ways a dog can turn out aggressive,
Being terrified - ex breeder dogs is a class example
Status symbols - idiots train their dogs to bite and be vicious to scare others
Dog fights - pretty self explinatory
Non socializing - this is a heart break of a one as it can happen to a good family, but if a dog isn't socialized with other dogs or other people, it can be vicious as a form of protection
I remember one time volunteering in my local vets, and this border collie came in. A bitch, only two years old. The very first thing we found out about it, was it was a vicious little cunt. It adored the mother and the 9 year old daughter to bits, however anyone else could not approach this dog. The father couldn't as it had already bit him, and it snapped multiple times at the 12 year old brother.
We had to escort this woman out of the room, get a dog catcher, use it, and heavily sedate it to give it an examination (it was also booked in for a spey). We had to sedate it, to give it a brief examination, as in check ears, teeth, eyes etc. It was insane. We put it in the kennels ready for its op,
Two hours later it came around, the vet owner wouldn't let any of the nurses or female vets help with it (he's a bit old fashioned, but lovely bloke), hell he didn't even let the newly qualified vets help, It was purely, him, a farm vet and me, we had to grab the dog with the catcher, wedge is between the door and a wall (imagine a open door being pushed against the wall). The head vet held it down with this catcher, the farm vet held the door, while using his legs as a brace against the frame, and I had to go in and stick it with the injection in the hind leg.
Needless to say after the op, the official advice to the owner was, give it up to a rescue who specialize dog behavior training or put it down. It is the only animal I've come across in 10 years of work / experience I've ever turned around and said it should be put to sleep. Believe me I've seen some shit shows,
And because I know someones going to point out "Oh you volunteered, BS you wouldn't be allowed near it etc etc", for context, the head vet is 6 foot something, former african military trained / vet for the K9 units they had aka he's as strong as an ox, the other lad works with bulls on farms and I teach both (or used to teach both) martial arts, and can put them both down on their arses, which in the head vets mind, made me more suited to help them than their staff. Also lets face it, my hands or fingers get bit, I just can't use a keyboard for a while, his vets hands get fucked they can't do surgery