Securing the scene does not mean serving as judge, jury, and executioner and that's what happened in this case. It should be disturbing to everyone that all someone hast to do is call the police and make an accusation against you and the police respond by immediately blowing you away.
This was not an execution, it was a justifiable use of force in self defense of the officers. You are not required to grab a judge, select a jury, and convene a trial at the time a felony criminal assault (and that's what point a gun at people as Tamir rice did is).
The "toy" he has was a replica of a real model firearm in colors that make it look real, with no orange tip, it would be impossible to tell the difference without inspecting it.
Someone called the police and informed them that a guy in the park was pointing a gun at people, that's the information the officers had, they find individual matching description of gun toting suspect, gun toting suspect then (instead of putting his hands in the air) reaches for his waistband, where after being shot, officers located the replica gun. (stop saying toy, this was a replica firearm)
The time duration is not unreasonable for the circumstances. any action towards a firearm may be immediately reacted against. if you wait to see them pull the gun the last thing you will see is them firing it at you, and that's the training assumption. you cannot draw a handgun from a holster faster then i can draw one I already have my hands on. it's not possible. hence once the hand goes down to the waste the clock in a high speed game where the stakes are possibly your life just started.
as a side note although it really doesn't matter from the legal side, Tamir Rice was 195 pounds and 5'9'' I'm 187 and 5'4'' and I'm 23. when I was 12 I was 110 pound and 5'2'' and the tallest person in my class. the point here being, Rice was abnormally large for his age, and it was completely reasonable for officers to assume they were confronting someone much older.
The only real reason this is a controversy is because Rice was 12. if I described this same script to you describing the perp as a 24 y/o no one would have an issue with it. Deadly force law is keyed on the reasonableness of the response, viewed through the eyes of the officer knowing what he/she knew at the time they pulled the trigger.
Tamir Rice's age and the 911 caller's speculation that the gun was a toy are irrelevant. the only facts that may be used in this discussion are
1) Someone called 911 describing an act of felony assault w/ a deadly weapon
2) this individual is the likely subject of the 911 complain
3) likely subject of Assault w/ deadly weapon complaint is reaching for waistband where most people keep deadly weapons.
deadly force is thus legally justified. And another point about self defense/ use of force laws, nothing in any law requires your information or belief's to be CORRECT for your actions to be justified, only that they be reasonable.