Really people?  A dog known by animal control and "it's owners" to be excessively aggressive.  A yard with no fence visible.  A man and his 3 year old, new to the street with no knowledge of the dog.  Premeditated,  Really?   

The average dog runs at about 25 MPH.  That means it would take 1.3 seconds for the dog to cross the 50 foot yard.  How fast are your reflexes and how much do you desire to protect your children and self?

Losing a dog is not fun, been there!   However once the blood lust of the offended neighborhood cools, life will go on and the former dog owner can get another dog to train to be a guard dog.  Hopefully he will keep it better restrained or better trained!  Perhaps in a few years the new neighbors will find a better and smarter neighborhood to move to anyway.