In the days before the Internet and body-worn cameras, it was easy for thugs wearing badges and carrying guns to get away with murder. Nowadays it's not so easy. Perhaps somebody should have told the Memphis Murderers.
I had a job involving the deployment of evidencing technology, not cameras, but motion sensors. You would be surprised at how many people carry on their nefarious activities, even when specifically told that the technology was being installed.
Often, the people who had the most to hide were the most resistant to having the tech installed.
You would think that the body cameras and security cameras would be a deterrent and for some it may well be. In my opinion, those with anger issues and violent tendencies need the satisfaction of acting out those urges and that takes presidents over the fear of being caught.