Every job has a learning curve, cops included. Departments don't just throw rookies on the street to flail away, they partner them with experienced cops. How much of what you're quoting has to do with ordinary turnover?
The question is why is there an increase in shootings and murders and to say it's due to BLM or antifa is simplistic. How are these increases broken down and what about all the strain on people due to the pandemic. It's complicated.