Again, to play devil's advocate-

One main problem with police being "held accountable" is that the public's perception of the events and the actual procedure/law involved are completely different.

One great example of this, while not specifically concerning police, was the Zimmerman trial- where, despite pretty much all evidence suggesting the shooting was in self-defense, a huge percentage of the population(especially the black population) were demanding Zimmerman's head- even though his only real crime was following a suspicious person.

In other cases, it's much less clear whether or not someone needs to be punished due to how the procedure was executed and what the laws are. Eric Garner's case is a good example of this.

Sometimes, such as the most recent case Philando Castile, it's incredibly clear that the officer was at fault and should be punished.

While people should certainly condemn the latter, expecting them to condemn the two former's isn't exactly fair.