The argument is that really bad dudes often carry weapons and police responding to a crime have to decide in a fraction of a second if their life is in danger, or not. If they decide wrong, and the perp is armed and uses it, they are dead. There's a reason why big city police work has such high rates of substance abuse, divorce and suicide. The stress is incredible for those on the street in high crime areas.

That said, are there bad cops? Cops who get off on their authority? Bullies? Prone to excessive violence? Of course there are. The public service unions make it extremely difficult to get rid of them, as does the tendency to close ranks and defend one another, even where administration tries. I think the TV show Blue Bloods does an excellent job portraying this.

Last, from your comments you would think there is an epidemic of white policemen killing black men. While one such unjustified killing is too many, compared to the millions of civilian interactions that occur daily, the percentage involving any kind of gunfire is very small.