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.