I thought it was to make him look more intimidating to have his hat off. Kind of like when Horowitz takes off his hair net when he's about to dump a barrel of trash on Viking (who happened to be played by Clancy Brown) in "Bad Boys". But maybe what one of the other posters has stated is correct - so the hat wouldn't fall off in the heat of giving the unfortunate offender a beating.

I was thinking the violent action he was about to perform might make the hat inconveniently fall off or fall over his eyes. And so he takes it off to prevent that.

It's not like the caps are strapped on. Issuing a beating may result in the cap flying off in a humorous display to both the prisoners and other guards, or perhaps even landing on the bleeding perp and soiling it.
Is very bad to steal Jobu's rum. Is very bad.

All good points. I don't usually analyse films, like this. It's just something i've noticed, every time i watch it.
James?
I think it may be time to go home
take me round the world one more time
Why not!

your vile!!!

I was being facetious! I'm not particularly interested in your community college psychology!!
We have to show the world that not all of us are like him: Henning von Tresckow
.

It's a psychological thing, like men taking off their wedding band when they do or think of something that they think is incompatible with marital fidelity. The ring represents the wedding vow, and the cap represents his duties as a prison guard. Taking off the ring, or the cap, creates the mental distance from these obligations that's necessary to do something which violates these obligations.

So in a sense, he was putting his official duties - represented by the cap
And the rest of his uniform?
We have to show the world that not all of us are like him: Henning von Tresckow
.

It's like a tiger's roar: To get into your head and make the victim afraid.

My interpretation was that Hadley knew he was not legally allowed to beat up inmates in spite of his position as captain of the guard. So in a sense, he was putting his official duties - represented by the cap - aside. After the beating he would return to the dutiful exercise of his job and put the cap back on.

That's a good point.
James?
I think it may be time to go home
take me round the world one more time
Why not!

I think he took it off because he knew it would fall off during the beating anyway. So probably just trying to keep it clean.