I played a number of shooters when I was young like this kid and continue to do so today. I was also around guns from an early age; I would go on hunting trips with my uncle and help him field dress deer. It made the damage guns can do very obvious. Now, despite having access to guns, it never would have occurred to me to take one of those guns and use it to shoot a relative (or anyone else).
I'm guessing this kid was either ignorant of the damage guns can do or he didn't care. Either way, you can't blame Niko.