You really don't understand root cause. This child was in danger no matter WHERE he walked. He was unattended. That's IS root cause.
It is unfortunate someone bad happened to the child. His parents should face penalties for such neglect. Even the city isn't taking the stance you are. They aren't banning ALL dogs, although all dogs have similar capability.
And the kid could have been in the house all day and that dog would have still eventually killed someone.
I dont understand how you feel like the kid was the root cause when you can remove the kid from the equation entirely and you're still left with a dangerous animal that WILL attack at some point. The animal doesn't just stop being violent if nobody walks into the yard.
That kid could have been hit by a car and it would have been an accident, but that dog killing the child was no accident on the dogs part, it was just waiting to attack. The dog had malicious intent, someone driving a car doesn't. That matters.
It's like being robbed and shot at an atm late at night. Yeah, it probably wasn't a good idea to go to the atm that late, but the root cause is someone looking for a victim. Regardless if you stayed in that night, that person was gonna rob someone eventually.