This. And it's way scarier than brutality. If the cops don't cover for each other and they can't file false reports, you can usually avoid getting roughed up or shot by not getting physical with them (although recent videos show for certain that even that's no guarantee--it just protects you from malice, not incompetence). Once they start filing false reports and backing up each other's lies, they're effectively beyond any control. They can do literally anything and get away with it, and a force that has unlimited power and no oversight will attract and eventually be dominated by people who will abuse it. That kind of culture is what turns healthy democracies into pre-industrial hellholes and keeps pre-industrial hellholes from ever developing into healthy democracies.
I'm willing to cut the officer a (very) small amount of slack here. People are calling it a "cold-blooded" shooting. It looks like more of a hot-blooded shooting. They'd been struggling and he was amped up. Hitting the guy with the TASER and having him not fall probably scared the hell out of him. He wasn't able to handle himself properly and he did a very wrong thing. He should answer for that just as any of us would answer for it if we shot somebody after a fight. But falsifying the report? That's fucking cold-blooded. Planting evidence (if that's what that object is)? Terrifying. I watched the video and was distrubed by the shooting, but casually dropping an object next to the body and calling in that he had a weapon? That gave me chills. That's the sort of thing that should be a capital offense if anything should. That's a direct, premeditated attack on civilization. None of us are safe.