I feel like I'm giving too many likes/agrees to stienman, given that I technically don't agree with him on many related issues.
A 12 or 13 year old guy, waving around something that looks like a real gun, is, and should be perceived as, a credible threat; this could just as well be a 25 year old guy waving around a Certified Definitely Real Gun, there can't be - and shouldn't be - a difference in how they handle it. A 12 year old waving around what appears to be a real gun should very much be arrested, brought to the police station, perhaps even interrogated if there's a chance he was really meaning harm (which wasn't the case in the TR case), perhaps even sentenced to something (community service, some sort of penalty to make him fully comprehend waving guns about in public places and/or pointing them at police officers is not okay).
Even a 25 year old, previously-convicted-of-murder, drug dealing scumbag, waving around a 100% Definitely Assuredly Real Gun, shouldn't have been shot without a chance to put down his weapon and surrender.
The police in the TR case didn't just randomly shoot down a 12 year old playing with some toy, who did nothing wrong, who was a saint. They didn't respond to a call about a 12 year old playing with a toy. They responded to "might be a juvenile" "brandishing" a "firearm". Given that, they still reacted wrong.
TR gave the impression of being a "possibly minor or perhaps not" guy with a "possibly real gun", waving it about and possibly endangering innocent people and the officers. If they'd drawn weapons, pointed them at him, called for him to put the weapon down, and arrested him, that would still have seemed a ridiculous over-reaction to "a kid with a toy" to some people, but they'd have been, IMO, in the right. If, after ample time and possibility to put the gun down, he'd instead have pointed it at the police and gone "bang bang, haha silly cops I gonna kill you", they might even have been in the right to shoot him - no matter how tragic and how much of a misinterpretation of the situation that might have been.
As it is, given they didn't give him time to surrender, they're clearly in the wrong. Their fault isn't, necessarily, "murder 'cause it's a black kid lol", but perhaps more in the realm of "overreaction to a possible hazard/danger", though.