This episode is revealing of what the late Jeff Cooper called "hoplophobia" -- an irrational fear of guns. Most sensible people realize that guns are inanimate objects, with no will of their own, and while they can facilitate bad people doing bad things, it's also true that good people can defend their own lives and the lives of their loved ones with them. Like anything else, guns can be abused, but the blame belongs with the abuser, not the tool.
Unfortunately, Jack Lord was a hoplophobe, as were many others who worked on this show, and hoplophobes, having an irrational fear, cannot regard the gun as the inanimate object it is; instead, it becomes something sinister and altogether evil. In this episode, the gun around which the plot revolves is exactly like The One Ring from Tolkien's "Lord of the Rings" trilogy -- they are evil in and of themselves, and just as The One Ring inexorably corrupts anyone who possesses it, it mysteriously transform their bearers into agents of evil. In this episode, anyone who touches The Gun becomes a ticking time bomb, destined to cause death and destruction sooner or later, utterly powerless to resist the evil of The Gun. People who were ordinary, law-abiding citizens, who'd never hurt anyone, are transformed by the corrupting influence of The Gun into dangerous killers, endangering the lives of others through their malicious intent or their recklessness. I'm not exaggerating. That's how this episode portrays it.
Hawaii Five-0 was a good show, with top notch production values, incredibly beautiful locations, and a charismatic star. At it's best, it could be a great show. But when it gets preachy like this (not to mention irrational), it descends into farce.