On the point of genetics - absolutely. The breeds that pit bulls descend from were bred for bull-baiting, not dog fighting, and some pit bull lines today are bred to be friendly companions - but most people don't know about their dog's ancestry to any level of detail. If you get a pit bull from a shelter, the streets, craigslist, etc. you have no clue if the dog was bred to fight or not.

Another issue that compounds the problem - since so many people are into "rescuing" pit bulls, many people are adopting dogs that have been shown some form of abuse or have a fighting history. Some dogs cope with it, others become unpredictable down the road.

So many people assume pit bulls are sweet because they've met one that's friendly. Many pit bulls tend to be human-friendly but dog-aggressive. This is another thing people don't expect when they adopt a dog with an unknown history. Many people who adopt dog-aggressive dogs either don't know how to deal with it or don't find out until it's too late. Crazier yet, some find dog aggression acceptable as long as the dog isn't attacking people.

But my question is this: why are people quick to understand that a human who harms animals has the potential to harm other humans, but slow to realize that a dog that harms other dogs has the potential to eventually harm humans as well?

I don't hate pit bulls, but I work in Detroit and they're a HUGE problem here. Plenty of them are friendly, happy, well-trained and well-loved pets - unfortunately there are many others that are a threat. I personally wouldn't adopt one knowing there's a high probability that any shelter dog in the immediate area was either bred to fight, trained to fight, or both. But I do wish the best for people who own pit bulls that are great dogs.