You've said that, but you're wrong - not even the police officer who shot him claimed that he was holding it in his hand:
Cop says he told Tamir Rice to show hands before fatal shots
Again, that's baloney. Do you want me to find you examples of police who've tried to talk down armed individuals who've posed an immediate threat?
ETA: Here's the FBI's report on the constitutionality of the shooting:
http://prosecutor.cuyahogacounty.us/pdf_prosecutor/en-US/Tamir Rice Investigation/Crawford-Review of Deadly Force-Tamir Rice.pdf
I can't find a link to the local prosecutor's report, else I'd post that, too. Note that, in defending the officer, the investigator said:
While I think this investigator's logic is ridiculous and part of the problem with police culture and our deference to them, it doesn't negate that even she doesn't claim that Rice was brandishing the gun.
So, your understanding of what happened is wrong. Your assessment of police ability to try to talk down armed suspects is wrong. We haven't addressed the fact that, if the officers truly thought he was a threat, then driving up to within 10' of him was pretty stupid. There's also the fact that Loehmann's statements regarding the amount of warning he gave to Rice were entirely fallacious. Why are all your errors skewed towards excusing the police?