There's no doubt the job of an American cop is hard. But you do seem to have a problem in the behaviour of a lot of police people.
At the same time, I don't doubt the difficulties of producing change.
The heart of the issue is that the police have to deal with a society in which the citizenry is on average heavily armed. Not just crooks, but also crooks. And nuts. And angry people. All the excuses and the bad operating procedures flow from there because they are the folks that have to deal with the situations they encounter. And many of those situations will be high risk.
Changing gun law is virtually impossible in the US. So I am not sure how you improve the situation without making their job even harder.
But yes, train them not to beat up innocent black men for no reason!!!