Not being US resident or citizen I wonder if this is the effect of the gun availability. Just few weeks back I was listening to a podcast where one of the guys went to US for couple weeks for vacations. They discussed the difference of the approach of the police.
Where I am, it's usually silly, when cop stops my car he salutes in overly formal way and in really angry tone (like really really angry) described what I have done and if I comply if not. Probably 99% of the danger for the cop him/herself is that I'm going to curse him and his family 3 generations back.
Whereas in US, it's all about police-safety first. They will scan back of the car, will ask (demand even) absolute compliance. This makes sense (to some degree, as police person is protecting their own life) since I could have a weapon and be a criminal or maybe even insane enough to try to pull this stunt. After living with this possibility for a while and having to estimate situation dozen if not hundreds time each day it's not surprising that police killings do happen.
613 of those 1089 killings in 2019 (56%) were gun related. And that's 75% of the "victim armed" cases. In contrary only 119 victims were confirmed to be unarmed.
That's probably something completely different than the authors wanted to communicate, but I think it shows how complex the matter is and undoubtedly can't be attributed to only single class of the issues.