I don't attach a lot of higher social meaning to this. In most crimes that are this disgusting, it comes down to one person who has mental problems who drives the act. Those problems were likely made worse by their home life, but the person themselves is just a sick individual.
If I had to speculate I'd say the 17 year old was a sociopath, with mental issues, and the 14 year old was in the same social circumstances and stupid and decided robbing people was the right thing to do. One of them (I'd speculate the 17 year old) surprised the other with his craziness by shooting the kid between the eyes.
We've had similar cases in Tacoma, my hometown. Some gangster dude murdered three people (one woman, completely unrelated to any gripes he had) execution style over perceived sleights and a jacket he lost. Come to find during trial the dude was just a nutcase, calling himself Lucifer etc., determined to do evil things.
I don't attach this to the gang culture in general but more to mental illness, like the guy in Newtown. Crazies are among us white and black.