You realize there's a wide array of options between "Letting him go" and "Shooting him in the back", right?
Actually, the problem is, we lavish all sort of money on the police. In Minneapolis, the MPD is 20% of the city budget.
No, the problem is that we make it too hard to get rid of bad cops. Unlike an HR department in any other company, you can rack up dozens of complaints as a cop, and the Police Unions make it impossible to fire you. Derek Chauvin had 18 prior complaints of excessive force, and not only did he still have a job, but amazingly, they had him out training rookies.
We also have the problem that police departments don't communicate about their rejects. So you have a guy like Timothy Loehmann, who as fired from Independence, OH's PD for mental instability, but Cleveland picked him right up and he proceeded to shoot Tamir Rice for playing with a gun.