It's one thing if you have a dog, either the individual dog or the breed, that is calm and submissive and occasionally gets in "normal" dog fights. These are always very short and although they look and sound nasty, they are just a lot of growling and nipping and neither dog gets hurt since they are just trying to establish who is dominant.
My mother's husky lays around all day, unchained and unfenced, watches other dogs stroll by and never goes after other dogs. The only time he gets in fights is when another dog comes into the yard and even then he is always friendly at first until the other dog gets aggressive. And he never pursues them once they back down, or he does, it's over. He occasional makes a tour of the neighborhood, and is friendly with the other dogs and people. I've even had neighbors come by to warn me and make sure he is at home when they see Animal Control on patrol. Fortunately, we live in a somewhat rural area outside the city limits and Animal Control is very accepting of loose but under control animals as long as it is apparent that they are at the homes of their owners. Their primary goal is to get the strays that people from the city dump out here on a routine basis.
All that being said, two pit bulls (an agressive breed) who normally have to be kept in a kennel being allowed to run loose, attacking a small dog, and shacking it to death is NOT a normal dog fight for dominance. This is a dog that was bred for hunting following it's instincts to kill. Owners have a responsibility to know the proclivities of the breed and to take precautions until the individual animal has shown a long history of acceptable behaviour. There is a balance between "It's all the owners fault" and "All dogs fight". Personally, I think the "First bite free" type of law is a reasonable balance. As much as I love my Mom's husky, if he had done this type of "kill the prey" attack, that would be sufficient proof that he was a danger and should be put down. There is a limit to how much, and what type of, "animal behaviour" can be tolerated as normal before it becomes an unacceptable danger to others.