and the grand jury was like "nah, cop did nothing wrong".

Reminder that the grand jury is just an arm of the prosecutor, and is frequently used as a scapegoat when the prosecution doesn’t want to indict someone. There’s an old legal joke that if the prosecution wanted them to, a grand jury would indict a bologna sandwich for murder.

Basically, the grand jury doesn’t have any defense lawyers, (because nobody has been accused of a crime yet, so there’s no way for a defense lawyer to defend against anything) and the prosecutor has full control over what evidence the grand jury is allowed to see. If the prosecution wanted to, they could just write “lol yeah I did it, signed [defendant]” on a scrap of paper, put it in front of a grand jury, and tell them to use it as evidence. It doesn’t matter if the evidence is legally gathered and admissible in court, because the grand jury isn’t a courtroom.

The prosecution can even exclude exculpatory evidence (evidence which specifically excludes the accused as a suspect.) Maybe there’s a receipt from a gas station across town, alongside timestamped CCTV video footage, which proves the suspect wasn’t at the scene of the murder when it happened. The prosecutor can exclude that from the grand jury’s evidence, so they aren’t allowed to factor it into the decision; The grand jury is only allowed to rule based on the evidence that the prosecution presents. Maybe the prosecutor knows this person is too poor to afford an attorney, so they’ll throw the charge at them (even though they know the person didn’t do it because of the exculpatory evidence) just to be able to say they closed the murder case. Because they know that when the person’s case lands on a public defender’s desk, the public defender won’t have time to argue it in court; They’ll just tell the person to take a plea deal for a lesser misdemeanor charge. The innocent person’s life is ruined for the foreseeable future, (because now they’re financially buried by the misdemeanor fines and the six months of jail time,) but the prosecutor got to cross another case off of their to-do list.

The grand jury is just a shield for the prosecution to hide behind, when it would be politically inconvenient to decide whether or not to prosecute someone. Prosecutors don’t like charging cops, because the cops will just outright refuse to gather future evidence for them. But the district attorney is typically an elected position, and the people are outraged at a cop murdering someone. So to save face with the cops and their constituents, they just don’t bring any evidence to the grand jury. This allows them to avoid prosecuting the killer cop, while simultaneously going to the public with an “I tried but the big bad grand jury wouldn’t let me indict them” sob story. Now the district attorney looks like a hero to the people for trying, and the cops are happy because the prosecution refused to indict a cop for murder.