I read the article and watched the embedded video. My 'guess' is this. Some scrote-bag youths brought along a pellet gun to drive-by and shoot homeless, bums, vagrants, whatever...to get their jollies. They probably thought it would just sting, and watch the victim/s suffer a bit. Perhaps an errant pellet instead entered the gent's eye-socket, or perforated a lung between the ribs, or an abdominal organ.
It's doubtful anyone would've heard a 'shot', in the sense of gunfire. Plus, the victim was known to have had mental issues. Would he have known he was shot)? Would anyone who knew of him paid him any attention if he complained, or outright asked for help? Brain trauma, Internal bleeding and/or oxygen starvation....