Teenagers don't have adult brains, true, but there are still some things they should unequivocally know are wrong. Shooting other kids in the back of the head at close range is one of those things.
I was a smart teenager. One day, a friend and I decided to go meet some older guys that were friends of her sister. We were 16 at the time. We ended up at the apartment of one of the guys. We thought we would be safe because we were "smart", and because my friend had a steak knife in her purse. In fact, when things got a little intense back in the bedroom with her and one of the guys, and he looked like he might not understand that "no" meant "no", she pulled the knife on him, and we left the apartment.
That was an incredibly stupid thing to do, all around. No one knew where we were, the guys were stronger than us, we didn't know them, and introducing a knife into the scene was just dumb. To our undeveloped teenage brains, though, it seemed to make sense at the time.
That's the sort of scenario I envision when I think about undeveloped teenage minds.
Or another recent example from the news, where this boy (a 10-year-old I think, he was young) brought a gun to school. He was going to run away with the gun for protection. He had it in his backpack, and when he threw his backpack onto a desk, the gun went off and a girl in his class got shot.
Do most 10-year-olds know that guns are dangerous and can kill people, permanently? I bet most of them do. But they're not adults, and I can definitely see how a child that age wouldn't fully understand why guns are dangerous -- that it's not just intentional shootings, that accidents can happen, that someone else could find the gun and use it in a bad way, or any of those other scenarios. They might intellectually realize that accidents can happen with guns, even, but not connect through the thought process to realize that an accident can happen with their gun.
Longer than I meant to type, but basically, though I agree that teenage brains are different and that teenagers are more risk-prone, have less impulse control, etc., acts like this I just can't see having anything to do with that.