When should you call the police?

When you are absolutely sure that by calling them you won’t be sentencing an innocent someone to death.

Whenever a crime is just happening.

Those darn facts.  The boy's mother didn't "call for help", she called 911.  In case you didn't know, 911 is for life threatening emergencies, not because your kid is pitching a fit and you can't control him.  Mom told 911 the boy was armed with a weapon.  Mom told 911 the boy was dangerous.  Now how about asking your question again with these darn facts included. 

The area cop friend calls all in he see's. & does slander for privlage's from the cops. So him.  As you say to get some one beat or shot for no good reason. To get a cop setting were you want to do a crime to move to the other side of its patrol area. Many insurance companies demand you get a cop report so forced to call them. But for safety never is best.  No need to risk your life around such.

Interesting conclusion based on the facts.

I agree.  You specifically should never call the Police.

Otherwise, not the least bit clever - "police thought it best ..."  Sad.

Well, to be fair, if she called 911, it was the 911 operator who conveyed the information to the police, and the police dispatcher who communicated to the cops who were sent there.  The cops were told there was an armed person threatening others.

I have little doubt that your version of this story is missing important details. Give us the full truth, not some watered down version.