It is called entrapment. An arguable police tactic I thing should be illegal. Like a police officer challenging you to a drag race, then arresting you for speeding. Nothing to do with deadly force.
Is it an effective tool? How out-of-hand is the problem?
Remember that guy in NYC that shot three teenagers when they 'asked' him for $5 while brandishing a screwdriver? A lot of people sided with the Shooter.