They have a very very loose standard imo. I kind of almost get it to an extent, but they can pretty much just say "I felt like I was in danger" and then anything after that is fair game. Tamir Rice for example. On the other hand, this Rayshard Brooks case is very very much in the grey area. The cops did pretty much everything possible right, util Brooks decides to attack them and take their weapon. Hindsight is always 20/20 so yeah, maybe they should have let him run away and arrested him soon after as he was technically not dangerous or armed. But in the moment, it's tough to be that reasonable and I have a very hard time feeling sorry for Rayshard Brooks.