To be fair the cop acted appropriately. You never know what the criminal is doing. YES the criminal was in handcuffs, YES he was in the car. But everyone has seen those videos of people escaping out of cop cars. And lets be real here he was already under arrest, IE he was likely a waste for society anyway. Its a fact and scientific evidence that criminals are always up to no good.
But it could have been the criminal. Its like Schrodingers cat. The moment he looked he would find out but at that point its better to shoot and be safe. I prefer a hardworking cop over a criminal. People have value in a society.