I linked to this in another thread. He clearly had an adult body (6'6", 270 pounds) but the mentality of a child. He needs to be held responsible for the violent crime he committed especially as he continued to beat the elderly female teacher after she was unconscious but looking at how he punch it is clear he doesn't know how to fight and instead relied upon surprise (attacking her from behind) and his weight to push her down rather than any skill.
Reading more it is unsurprising he came from a broken home with a toxic racist single mother who probably taught him white people are the source of all problems for black people. He is a lost cause so a decade in prison would be a fair sentence in my mind. Sadly, a 17 year old like him would get destroyed in prison but that is the life he picked. Though, to be more honest, he never had a chance given his shotty racist toxic single black mother.