All officer involved shootings are investigated, but you take issue with the investigation being internal. If an independent investigation office was created, would you accept the results if the results still were the same? What group, would you accept results from in independent office from as not compromised or part of a pro-police conspiracy? Certainly not any Trump appointed office, no matter how seemingly neutral.
Also, circling back - even if the police only killed violent people, my personal experience is that their interactions with the community are pretty much always to threaten violence - except for their attempts to indoctrinate children into accepting their presence. I've seen police break ribs and knock non-violent, merely uncooperative, people unconscious, then fail to provide healthcare. They've harassed and threatened people attempting to film them, including taking details and arresting people who refuse to provide them. There was a spate where they arrested random people on my street for no stated reason, then released them just before they had to explain to a judge why they were arrested. Just because people don't die most of the time doesn't make them unaffected.