A little hitler is a self-important tosspot
who thinks he's in charge. Someone who makes up arbitrary and/or self-serving rules and has a tantrum if they aren't obeyed. They tend to have a park-keeper/traffic-warden mentality; rules are Law and rules come first. They can't handle people who threaten their authority.
Wayne's running a tournament..
Wayne: You didn't spell the name right, Jim. You're out.
Baz: That's unfair, Wayne. You know what he meant.
Wayne: You're out too!
Baz: Ya stupid little hitler.