I had to tell someone that, yes, Japan has had mass murder incidents in modern times. And that, yes, a lot of them have been against defenseless old people, or sick people, or developmentally disabled people. And yes, a lot of them have been with knives or swords. (Although gas and poison are also favorites.)
Probably the worst mass murder in Japan, in a true crime context (as opposed to war crimes or political crimes) was an incident where an unpopular young guy cut the phone lines out of the village, decapitated his grandmother, and then killed all the inhabitants of his entire small town of Tsuyama. This is known as the Tsuyama Incident, Tsuyama Massacre, or Tsuyama Jiken (murder case). He used a katana, an axe, and a shotgun, before killing himself with the shotgun. This was back in 1938… but yeah, it’s not the only mass murder.
So yeah, even though Japan’s overall crime rate is low, there is a fair amount of gruesome true crime, and even mass murder. And no, the draconian gun control laws do not stop even gun crime from occurring, much less knife crime and sword crime.
I feel like I’ve kicked a puppy, when I tell Japan fans about this sort of thing. But it’s dangerous to go around with false ideas about crime.
Aliens in This World 
I also had to tell someone that Native American tribes slaughtered each other and warred upon each other before any Europeans showed up. Man, it is like kicking a puppy… away from poison.
Yeah, truth matters.
….
Would it help them any if you told them that most of the stopping-gun-crime stuff is done by their organized crime?
It’s highly over-simplified, but you don’t want to use a gun in a crime because the Japanese mob will kill you, and make it ugly.