There have been plenty of (questionable) psychological and sociological experiments that have shown that most people are perfectly capable of evil in the right (or wrong, I suppose) circumstances.
That means the only way to prevent evil is to shape the circumstances in such a way that encourage people to do the right thing, and discourages evil.
So with police, you can't merely trust them that they will always do the right thing and not mistreat or murder suspects, you have to train them, hold them accountable, and create a culture where bad behaviour is simply not acceptable.
My impression of the US police (from reading these kind of stories, as a foreigner, not having any first-hand experience with it) is that US police frequently fails on some, and sometimes on all three of these issues.
That means the only way to prevent evil is to shape the circumstances in such a way that encourage people to do the right thing, and discourages evil.
So with police, you can't merely trust them that they will always do the right thing and not mistreat or murder suspects, you have to train them, hold them accountable, and create a culture where bad behaviour is simply not acceptable.
My impression of the US police (from reading these kind of stories, as a foreigner, not having any first-hand experience with it) is that US police frequently fails on some, and sometimes on all three of these issues.