A Colorado dog owner accuses another man of killing his pit bull after their dogs scuffled at a park
First, there is no justification for taking the law into your own hands. Second, this isn't how you break up a dog fight. Third, we don't know which dog started the fight, and just because a pit bull was involved doesn't mean that was the dog at fault.
At off leash areas, you need to watch your dog closely, know your dog's tendencies and the other dogs that come there regularly, and have a plan on how to break up a fight if one should occur. I do believe dogs should be allowed off leash areas in cities and in the country, but you have to be wise about when to let your dog off leash.