I was just thinking about Trump's admonition not to treat suspects gently when placing them in custody, and how minuscule are the chances this would have an impact on crime rates. But then I realized it didn't matter. External reality is simply not the milieu in which Trumpian politics operates. It's all about provoking certain emotions in people.
In fairness, a lot of law enforcement officers are Republicans who feel feel like Obama attacked law enforcement and who feel like Trump represented a sea change from years of disrespect. The notion that those perceptions led to an actual change in violent crime rates seem a hard pill to swallow, especially if we are comparing 2019 crime to 2020 crime.
I'm not at all surprised. You're seeing a period of mass unemployment, millions in financial crisis and huge political instability. An increase in violent crime would be predictable under any leader.