I reported my boss to HR based on his inappropriate behavior (with other employees at the company). I was put into the next round of layoffs. Needless to say I don't trust HR anymore.
If you feel like that would be overkill, don't report anything to HR. They don't work for you; they are not on your side; they will never be on your side.
If it is an issue that could be criminal, hire an attorney and have them tell the cops, without letting HR know about it first. Internal whistleblower programs are probably a trap.
In short, nobody should trust HR, ever, except the board and CEO.
But they're not on the manager's side either. What rational HR department would want to keep a manager who makes their subordinates feel unsafe, work ineffectively, etc?
Any of them, because:
(1) The manager is more likely to know their legal rights,
(2) The manager is more likely to be able to afford the cost of litigation in a dispute,
(3) The manager is more likely to have a contract beyond at-will employment,
(4) Determining whether the manager is a problem or the employee is malingering has a cost, plus risks creating evidence of corporate liability
Etc.
Except in the most clear and egregious cases, HR is naturally aligned with management against the human resources that management manages; that's kind of the point of HR, whose function is to assist management in efficiently (i.e., at minimum cost per unit value produced) managing those the human resources.
HR is not a neutral referee; the closest there is to that is government labor relations authorities.
Sure, it might work most of the time. But the one time it doesn't work, could cause billions of dollars in damages.
Just look at what happened to Uber after they retaliated against the wrong "low level" employee.
Because they reported them. The rest of the employees are not reporting anything, and are therefore not a problem for HR to be dealt with. The conflict is between the reporting employee and the manager, who has more political capital. Therefore, HR sides with the manager to minimize risks to itself.