One reason is money...this killing saved the justice system a fortune and saved the U.S. taxpayer the roughly $50++ grand a year that his eventual incarceration would have cost.
No doubt saved a few lives of potential victims who would have somehow annoyed this “child” or didn’t move quickly enough. “You got it, I want it, and I’m taking it with this gun”. Wonder what Jesse Jackson’s reaction would be in this situation? He who said he was relieved when the faces of the footsteps behind him at night turned out to be white. His words, not mine.