Bad guy with a gun normally gets it by stealing it from a good guy with a gun. The black market fully depends on stealing guns from legal owners.

Well, Japan it's from the gun control. You have to prove you can actually aim the gun. You have mental health checks. You are required to use a safe and you get a frandom inspection every few years to make sure you're actually USING the safe. So a kid taking dad's gun and shooting 9 kids is unheard of. Some years they have 1 shooting.