I have absolutely, positively observed different behaviorial tendencies based on breed. My Beagles won't fetch. My Chesapeake Bay retrievers won't stop fetching. My Beagles won't swim. My Chesapeakes won't stop swimming. My Beagles will put their noses down and follow a scent to the end of the world. My Chesapeakes will use their eyes along with their noses.
Working breeds (the non-cosmetic ones) have generations of 'un-natural selection' behind them. Some were bred to retrieve, some to trail by scent, some to bite sheep on the heels (ever watch a blue heeler?). Unfortunately, some breeds were developed to fight. The genes that were selected to produce the fighting characteristics are still present in these breeds.
This doesn't mean ALL Pit bulls, Canary Bulls, Dobermans, Burnese mountain dogs.... (etc) are viscious... or killers. I know animals from all those breeds that are complete sweethearts. What it does mean is that the 'wiring' for aggression is still there and can be activated more easily than other breeds. When that happens, things can go horribly wrong in a big hurry.
So, I have mixed feelings about the 'fighting breeds'. I suppose the genetics of these breeds can be changed over time by de-selecting agressive animals, but that will take awhile, especially with the 'bad folk' still engaged in breeding for the fighting ring.
Dunno. Can see both sides. Wish I had a better answer.