After comparing the cops story and the video they did not know about when they shot the man in Dallas:
http://benswann.com/caught-on-tape-dallas-cop-shoots-mentally-ill-man/
I think I might be more open minded than you in such cases.
But, since we're expounding, here's my "theory". Boy is threatening mother with a screwdriver and having a "bad episode". Stepdad calls the cops as he has 20 times before to deal with the boy. Cops attempt to subdue boy, but boy flails around and refuses to relinquish the screwdriver, striking one of the officers. Cop fires, killing the boy. Stepdad initially says that cop says "we don't have time for this.....taze him." (there's video of this). Stepdad starts getting crap from mom about involving the cops. Stepdad deflects and later changes story to "cop says we don't have time for this and shoots boy". All to assuage his guilt at having involved the cops which ultimately resulted in the boy's death.
My "story" is just as plausible as yours. Neither of us knows, and all I asked that we remain open minded and not run off half-cocked and jumping to conclusions about police brutality.