I've been on both sides of the 'velvet rope' in my late teens and early 20s. I went out a lot as a student, but at the same time paid my way through uni by working the doors on the other nights I wasn't going out. It was a mixed bag experience that I'll probably never fully digest.
I was invited to work at a late night club unofficially cash in hand after my regular venue closed, and saw the 'deputy head doorman' there grab a guy who was obviously worse for wear but not doing any harm. He pulled him in to a fire escape stairwell and it sounded like a battering was going on. I don't know for sure if that bouncer filled in a defenceless guy who was in no state to defend himself, but heard enough stories to strongly suspect it.
The weirdest bouncer I ever encountered was female. Back then female door supervisors were a rarity. This one almost looked like a normal mid-late 20s lass, but was absolutely unhinged, ready and willing to assault anyone in the blink of an eye and obviously loved it, along with being a master of dishing out verbal abuse. I'm certain she only got away with it due to how startling being attacked by her was (and the fact her targets were too confused to really respond and chose to retreat), and the fact she always had multiple male bouncers right behind her. Her style obviously worked as she became head doorsupervisor at a large night club, which subsequently lost its license some years later because of a) murders by the public, and b) a murder or two by the door supervisors.
There were multiple other non-violent but obviously psychiatrically disordered types I met during my time as a door supervisor, including sociopaths who were masters of gossip and lies, some to the point of pathological lying. Disturbingly these were often the types that can really charm the public and the right 'high value targets' in the night club industry.
On the other side of the coin there were a bunch of guys and girls I worked with who were perfectly 'normal', stable, sociable, helpful, and nice but not a walkover. They treated the public probably better than they deserved and had no love of violence. One thing I learnt is not to judge door supervisors by their appearance, both male and female, as it often defies expectations finding out who are the sane ones versus who are not.