Obviously I'm only one person and my personal experiences only apply to me, but I'll present them as a single example. Use them or ignore at your own discretion.
I was raised knowing how to use a gun. As soon as I was big enough to handle the recoil of a gun my parents took me out and put one in my hands at a target range, I think I was 7 the first time I had to shoot. My family doesn't hunt, we don't shoot for sport, but my parents felt it best to have guns in the house for protection. They felt that it was better for me to know what those guns where and what they did and how to use them properly from the very begining.
Once a year until I was probably 14 we went out and renewed the lessons. And let me say, I HATE guns, I hate shooting guns, I hate the smell, I hate the recoil, I hate the knowledge of what that gun could do if I screw up.
Now imagine. I am not a pretty person (photographic evidence available in the things in the universe thread). I am short with fuzzy hair, a lisp and on top of it all I walk like a duck. Add to that I've been a nerd since I was very young, and grew up poor in a very rich area (Lake Tahoe). Was I tortured and humiliated as a kid? YOU BET! I've been pushed down, beat up, yelled at, laughed at, had kids follow me around my school with scissors to cut my hair off, you name it. And not just by the kids, you'd be surprised what adults will say to kids. All because I wasn't "normal".
So I am a person who was raised knowing HOW to use guns. Knowing WHERE to find easily available guns. And I was a social outcast humiliated by my peers and teachers. I fit the description of a dangerous kid.
Did I ever, even for a second, CONCIDER an act of voilence against ANY of these people? No. Don't get me wrong, I hated them. I cried for years. I'm SURE it's where my Social Anxiety Disorder comes from. But I would never, even for a second, concider hurting them. It's just not worth the toll on my soul to sink to their levels.
Sigh..I'm way off the point. I don't see it as morally irresponsible to teach kids about guns. Infact, I agree that in some cases it can be a benefit. I think it is very important that kids have some knowledge of guns. If not practical working knowledge at least an introduction. It's like cigarettes and sex, if a parent doesn't teach their kid you don't know where they'll get their information or the quatlity of that information. As much as I hated it, I did learn a lot because I was forced to use those guns. It didn't change how I value life, it simply made me aware of how dangerous a weapon can be.
I DO think it's the responsiblity of the individual to take the lessons of their life and make good choices for their future. The kid in the original story was in high school, more than old enough to know better. I would be worried if I saw someone using a book like that and writing "DeathNote" style messages in it. It would be very sad if the seriousness of his actions was overlooked because it's similar to a manga. I hope that this person is getting help now.
However, like I said. These are my opinions, based on my experiences. They don't always hold true for everyone.