Or Japan with arson. That was brutal.
You have a valid point that gun ownership is a constitutional right. But it has gone too far. One cannot yell "fire" in a crowded theater and cause people to be trampled to death. Neither should one be careless with firearms. Restrictions on constitutional rights have become a long held standard. "Shall not be infringed" is nice and all, but people get murdered and we suddenly have a conflict between the right to live, VS the right to easily take lives.
Although, I suppose Life, Liberty, and the Pursuit of Happiness isn't exactly something enumerated into the Bill of Rights.
Still, I hold that they are rights. And when two rights are in conflict, one right must prevail at the expense of another. Kids like this shouldn't just turn 18 and grab the means to kill. They shouldn't be guaranteed that right at the expense of people's lives. I'd require a license with specific training and screening, and I'd restrict to single shot bolt action rifle, unless otherwise professionally employed. But that's just me in my desire to uphold the lives of other people against the easiest weapons of mass destruction readily available.
I am for action, you are for... evasion? Does the stats quo suit you just fine?