He should have been allowed to speak out for anything, no matter what it was, without facing retaliation. It is tricky since as a police officer, he is supposed to be some sort of face for the public or for officials or something, meaning that him speaking out could lead to complications... but still.
The officer who wanted to testify about the roadside incident with Sandra Bland should be allowed to testify without threat or retaliation. The code of honor of police officers to protect each other, also known as the "Thin Blue Line," should not be used to protect officers when they have committed criminal offenses.
The Sandra Bland case is full of bullying and a terrible justice system that preys on the weak. This officer should have been allowed to testify. He should not have had to worry about his job or his life or anything else. Retaliation should not have been on his mind.
No, there is no need to threaten someone with retaliation. If he had information, why wouldn't he be allowed to share it in a court of law? Anyone with real information on a case should be encouraged to come forward with it, not threatened with retaliation. How else will jurors get the information they need?