[EDIT for clarity]: it's ludicrous that prankster and police officer aren't both charged. Police incompetence is no exoneration of Tyler's responsibility, and conversely, Tyler's criminal actions are no excuse for the police officer's lethal lack of judgement/restraint. Of course Tyler is a criminal, I thought it went without saying.[/EDIT]
The core problem is that it's OK for US police to kill US citizens, and it's essential not to shift the blame. By primarily blaming Tyler (who's a super-nasty piece of shit, but that's another issue), medias and justice reinforce bad police officers' sentiment of impunity.
In most developed countries, if a cop shoots someone who wasn't actually about to kill someone, he's criminally liable. His career is over, and his freedom in jeopardy. If he wrongly "felt threatened", that's poor judgment and incompetence, certainly not an excuse. And whether the victim was guilty of anything or not isn't a mitigating factor either: it's exclusively up to the justice to say so and to punish. That's why Judge Dredd isn't called Copper Dredd.