The cops killed a 12 year old in a park because he was carrying a toy gun.
Wrong as usual.
They didn't kill him because he was carrying a toy gun.
They killed him because they made a mistake.
Find a way to produce cops who never, ever make mistakes (i.e. superhuman robots), and this will stop happening.
But as long as cops are human, it will keep happening... very occasionally.
OTOH, if you disarm cops, or take them off the streets completely, a lot MORE people will get killed.
Is that what you want?