Police jobs are boredom punctuated by short times of great stress.
It requires great restraint and training. In case you haven’t noticed, some obvious evidence has been overridden in recent cases on police killing unarmed black men.
The prosecutor guides the grand jury, giving members the points of law to consider, in a closed system.
The system where a prosecutor who must work with police before and after the grand jury seems to be an obvious conflict of interest. The police will be much less likely to work cooperatively with a prosecutor who puts one of their buddies on trial. Also, grand jurors could feel a threat to their lives if they vote against a police officer.
By the way, the prosecutor can override a grand jury — he or she just has to go before a judge and demonstrate that there is enough evidence to put someone on trial; for some reason this hasn’t happened, in spite of obvious video evidence.
The incompetence displayed by the prosecutors might well have been intentional. Many states do not use this system of grand juries, rather they use preliminary hearings, which are adversarial, open to the public and involve a judge. Perhaps a closed preliminary hearing would be preferable.
For cases involving serious injuries requiring a doctor’s care or death caused by a police officer, federal agents should get involved, collecting and assessing evidence. During a trial, defense attorneys should take on the task of prosecution, avoiding conflict of interest.