Not just mall cops, it's just people in general in any position of power. When I was young I used to host game servers for a community I created and liked to have a decent amount of people to administrate them and keep the games fun for everyone. There were people playing for months and always seemed reasonable and level headed and I'd see if they would be interested and most jumped at the chance to be more involved in the community. Every once in awhile though those reasonable and level headed individuals once they got some measure of authority went absolutely crazy and there's no indication of who it would be. People can be the exact opposite too, they clown around taking nothing seriously always trying to push boundaries, but then you give them some responsibility and suddenly they are the most responsible person you've ever met, they just needed a chance to show it.

Depending on the state, security guards do have some power. In Tennessee, guards can be bonded, which effectively makes them cops.

In Virginia, security guards have powers of arrest, so they're not cops, but can legally arrest and detail you, to include handcuffing and up to lethal force in certain situations.

But to your larger point, it's a power trip. I worked security for 10 years. Most guards do not give a fuck, they don't want to do anything more than the bare minimum, and will passively just sit there while people steal and shit.

But occasionally you get a power tripper. Someone who went into security because they couldn't hack being a real cop, so they decided to become a rent-a-pig. This is usually seen in people 60+ or under 25.