Cop showed up and according to reports of eye witnesses the cop got out of the car and shot him within two seconds. But then, did the kid point that gun at the cop? That’s the question the DA will bring up.
After Ferguson I am hesitant to believe any witness who speaks to the news right away. That said I have seen no reports that the kid pointed the gun at the cops, only that he lifted his shirt. The choppy nature of the video makes it hard to be certain but in one frame the kid is pulling up his shirt and the cop already has his gun out and in the next frame the kid is buckling from being shot. There is some ambiguity but to me it looked pretty bad for the cop.
This article about the incident is interesting:
http://www.cleveland.com/metro/index.ssf/2014/12/tamir_rice_killing_caused_by_c.html