Police officers have a difficult job. Absolutely. They're put in situations, on a daily basis, in which their lives depend on split-second decisionmaking. Certainly. Police officers put themselves at risk for the good of the citizens. No argument from me. They should care about their fellow police officers and show solidarity in the face of adversity. Damn straight. There are nations that have it worse than the United States, and we should be glad that we don't have to deal with such blatant and state-sanctioned corruption, brutality, and general evil. Unquestionably. Mistakes do inevitably happen. Sure.
But usually, as a civilian worker, when you make a mistake that kills somebody, you have some pretty bad consequences for it. Often, you'll be fired. Usually, attempting to cover up the mistake gets you disciplined pretty severely on top of whatever consequences you have for the actual mistake. In fact, that's usually a lot worse than the actual error, since it's a premeditated, intentional act. You'd certainly be fired for that, at the very least.
I've heard all sorts of good arguments for why we should attempt to understand police perspectives and avoid demonizing them, and I agree. But those arguments always get raised in the context of telling us why we shouldn't even try to hold them accountable for their mistakes. Life shouldn't work that way. Sympathy should not buy you immunity to consequences.