This is the part everyone misses. I worked in HR for a number of years and 90% of my job was telling low/middle level managers "you can't do that to your employee." (I wasnt high up enough to be dealing with c-suite level complaintants), 9% was recruiting and paperwork, and 1% was telling an employee "You did something potentially terminable."

Most people only seem to recall that 1% and then keep talking about how "HR isn't your friend/on your side theyre on the company's side." Which is true! But they also didn't see the 1000 times I slapped their managers hand because I was on the companies side not the managers. Unless your really high up your manager is someone's employee too. HR isn't siding with you manager for shits and giggles, there is a reason management won a complaint against you and it isn't "HR likes management better." It's that they framed your problematic behavior better than you framed theirs. Frame everything you report to HR as "this is why it's a liability for the company" not "I don't like x,y,z. So-and-so is mean."

Also remeber just being a bad manager (not doing something immediately terminable) isn't a firable offense. Yelling/being a low level dick for example may not be something deemed firable. One complaint isn't gonna e enough and ideally multiple people will complain as well.