It’s easy to say there was no need to kill him with 20/20 hindsight (although we really don’t know that that’s true). But the law doesn’t use 20/20 hindsight as the standard in these cases. It uses the reasonable perception of the officer at the time. That’s why there’s a grey area.
The planted evidence thing is a distraction. He could just as easily have been securing a loose weapon so a bystander couldn’t get hold of it. We’ll just have to see on that one.
I don’t see any such checks to abuse of power in the model of policing that you propose. Are we subjects now, to what is essentially rule by whatever violence the government should choose to mete out?
If you were the subject of an abuse of power by an officer of the law or other agent of the government, what checks to such abuse would you want and/or expect to be in place to deter those abuses, protect you from them, and/or compensate you for damage done by them?
This question is not only generally relevant to the topic, but also specifically relevant to the jurisdiction, which has a reputation for excessive taser use, and this specific officer as well.