It sounds harsh to call cops who fail to report bad colleagues “cowards.” A degree of loyalty is inevitable among people who risk their lives with one another. They understandably fear being turned on by the most violent and unethical among their peers, retaliation from management, and the prospect of losing their livelihood. But cowardice is defined as “fear that makes you unable to do what is right or expected.”

The word fits."

https://www.theatlantic.com/politics/archive/2018/04/the-cowardice-of-covering-for-too-violent-cops/