I think the primary benefit of an electromagnetic coil gun in an environment like Japan would be the capability to build one using off the shelf and unmonitored equipment. In the US it would be much easier to generate 3d printed guns, because finding ammunition with gunpowder inside is much easier.

The translation of the article is a little rough. It looks like maybe it was used as a curiosity instead of a tool for violence. I suspect actual criminals get along just fine with melee weapons and the occasional smuggled firearm.

This whole thing is cool in that it feels very much like a 21st century style crime. It's scary because I imagine disassembly would be an effective method of disposing of the murder weapon. In most jurisdictions it's possible to get a conviction without a murder weapon but requires more supporting evidence.