Yes, police have actively been using black people as scapegoats for homicide that the police carry out for decades. There used to be a code police would put on bodies in the 80s and before - “DNE” do not investigate, indicating a homeless person or sex worker who probably will never be identified. However, it has been found, particularly with LAPD, that they were using this indiscriminately and many cops through the years were found to have covered their own murders with this code.

"DNE" is the opposite of inventing evidence to prosecute black folks for killings secretly done by police - it's institutionally telling everyone to drop the ball on this one. For "DNE" to be a significant part of what the statistics show while your beliefs remains also true, cops would have to be going around killing and "DNE"ing white folks (which seems backwards from what you'd expect from people driven by white supremacy) and/or have an elaborate conspiracy all the way along the chain from cops to coroners to forensic techs to prosecutors to judges to jurors to frame black folks for murders of other black folks.

Because again, for homicide and race statistics to be what they are because of white supremacist policing there either have to be a bunch of white killings not being counted as homicides at all, or a bunch of totally fabricated frame jobs for killings of blacks fabricated in a conspiracy that is shockingly tight lipped despite being utterly massive (something like a third of all US homicide to bring black homicide rates in line with share of population) and spanning decades.

More likely is what you find when you dig deeper - a huge proportion of homicide in the US is gang related and young black men are the primary gang recruitment demographic for a mix of cultural and mostly economic reasons.

Public shooters, school shooters, going postal shooters, were all typically white male shooters (the one black shooter I can think of was a cop).

Yes, public mass shooters tend to be white men, typically young ones. There's about 25,000 homicides per year in the US. Since the 60s, public mass shootings have accounted for a total of about 1,500 of those (not 1,500 per year, 1,500 total). And those shooters either die on the scene because the whole point was to kill themselves and take a bunch of innocents with them, or get tried and convicted.

You'll note I said public mass shooters instead of just mass shooters, because the difference is relevant. It's a trick of the statistics - when talking about how bad mass shootings are, people will point to Sandy Hook, Columbine, Aurora, Pulse, etc but when talking about how often they happen will include any shooting with three or more casualties. If you limit your talk of mass shootings to only include shootings that were not limited to a single private residence (such as home invasions and family annihilators, which tend to have different demographics) and were not done as part of some other criminal activity (a lot of those are gang or cartel related, and have different demos as a result), then what you're left with is mostly young white guys in dire need of mental health assistance who decided to take a bunch of people with them when they killed themselves (as in Columbine) and the remainder are mostly white supremacists who think they're acting to defend the white race (as in Christchurch) or some similar bullshit.