I am a dog owner, and I agree that a loose and aggressive dog is a liability to humanity and you are making everyone a favor if you kill it even if you could have escaped (e.g. you manage to get into your car in time). If it was attacking you, it could be a child (or another dog, too) next time. I would say that even a loose, non aggressive dog of a breed that is very statistically likely to mangle people (e.g. Bully XL) should be taken away from their owners that same day.
I think there should be mandatory courses the owner needs to take to own these breeds (and pass the exam with satisfactory marks), loose or not if one slip of a leash can lead to a kid getting mangled then it's not safe for the owner to walk even a leashed untrained dog.
As a fellow dog owner I feel obligated to point out that not all barking is aggressive.
People not used to dogs often seem to think it is, and I'd hate for people to start shooting dogs just because they bark. Dogs bark, it's what they do.