It's not just a maturity issue at 16 years old, it's a matter of lack of certain experience(s) upon which to draw from. You can be "mature' for your age and still lack experience in many, many things and still not be handle certain circumstances because of your lack of experience. At 16, while you are on your way to adulthood, you are still in many ways, a "kid". You don't have all the coping mechanisms/skills nor the same focus and retention values (which may all fly out the window during a stress situation or an emergency, or some unexpected situation that you are suddenly thrust into) as an adult generally does.
Maturity is a process and I don't care how mature you feel you were at a young age, when you look back in time many years later, you realize you were not as together as you thought you were. That's just natural, it happens to everyone. Confidence is another issue. As a teenager, no matter how you may appear on the "outside", you just do not have the same level of confidence as you will as an "adult", years later, nor will you have the same level of wisdom as a normal adult.
"Neuroscientists now know that brain maturation continues far later into development than had been believed previously. Significant changes in brain anatomy and activity are still taking place during young adulthood, especially in prefrontal regions that are important for planning ahead, anticipating the future consequences of one’s decisions, controlling impulses, and comparing risk and reward. Indeed, some brain regions and systems do not reach full maturity until the early or mid-20s.
First, different brain regions and systems mature along different timetables. There is no single age at which the adolescent brain becomes an adult brain. Systems responsible for logical reasoning mature by the time people are 16, but those involved in self-regulation are still developing in young adulthood."
http://learning.blogs.nytimes.com/2012/05/30/when-do-you-become-an-adult/?_r=0
We won't really know what happened or why until the investigation is complete so I don't care to speculate on what may have happened.
It's terrible that this happened to this young man, and my thoughts are for his parents and family.