When the state pays directly, there is no incentive for the individual to behave. If the state paid the officer's salary plus a standard 'allowance' equal to the cost of police malpractice insurance for an officer with no claims, then the incentive is to keep their record clean or pay higher rates out of pocket for misbehaviour.
Noone (neither police nor citizens) want them on the force as they make everyone's life more difficult. But right now there is zero incentive to remove them, and only a union fighting to back them no matter what.
I'm assuming if you have a stream of 20-30+ excessive force complaints against you over the past two years, based on past behavior you are very likely to have them in the future and be the cause of a terrible situation leading to a lawsuit.
If you have a clean record, your car insurance is low. If you have multiple at-fault claims, your insurance is prohibitive.
Why can't we do the same for professional services? The individual who acts ethically ends up with lower insurance than the individual who acts unethically. And at the same salary, the ethical person will take more money home.
That's individual incentive.