My SIL got a rescue pitbull, worked with it every day and it was very well behaved. One day they came home and there was blood all over the house. It had attacked their other dog and luckily only grabed the collar which was ripped off and the tags all bent up. Dog was unrescued.
Few years later, I went to a dog park with my poodle. He sniffed a pitbulls butt and the dog whipped around and latched onto my dogs neck completely unprovoked and without warning. It took minutes and two adult men to get the pitbull off. Got lucky again and the pitbull only grabbed my dogs scruff but he still has scars years later. If it grabed just a little bit up my dog would have died.
Just a few weeks ago, me and my wife were walking down the street with our two dogs and 2.5 year old. Some dumb kids are chasing their 3 pitbulls through the streets as they got out. The loose pitbulls see our dogs from a block away and sprint at them. One runs up to my dog attempting to pick a fight immediately. I have had enough at this point and physically remove these dogs over and over until I can get the kids to control their dogs (which the never really do).
A bird dog naturally points birds. A bloodhound naturally follows scents. A cattle dog naturally corals animals (and toddlers). Is it so unbelievable that a dog fighting dog naturally fights dogs?