Not much is known about Beni's past, just that he is from Hungary. In the 1920's he was part of the French Foreign Legion with Rick O'Connell, and the two were friends. When the legion made it to the lost Egyptian city of Hamunaptra, they were attacked by Arab fighters. Beni betrayed Rick and hid in the ruins for safety, locking Rick out and almost leading to his death until the Arabs were scared off by Imhotep's face in the sand. After this, Beni started a business of charging people money to be taken to Hamunaptra, only to abandon them in the middle of the desert after being paid.
He meets Rick again when leading some Americans to Hamunaptra, but this time has to take them all the way because they're only paying him half when they get there and half when they make it back safely. While in Hamunaptra, Imhotep is reawakened, and is about to kill Beni as he recites prayers in several different religions hoping one will work. When he reveals that he speaks Hebrew, Imhotep lets Beni live so he can act as a translator because Imhotep also knows Hebrew. From then on Beni becomes Imhotep's minion, not caring that it means the death of his supposed friends and the entire world being doomed if Imhotep wins. Beni becomes a victim of his own greed at the end of the movie, when while trying to carry bags of gold out of Hamunaptra he triggers an ancient booby trap that makes the ceiling collapse and traps him in a room filled with flesh-eating scarabs, which presumably eat him alive.
Beni is a lying, cheating, cowardly crook. He has a habit of running from danger, betraying his "friends", or what he considers his friends, and even joining, what he considers to be, the winning side just to stay alive. Beni is also shown to be extremely greedy and rather stupid, as shown at the end of the movie. Beni tries to grab as much gold as he could before he tries to escape the collapsing tomb, but as a result, the tomb closed in on him, leaving him sealed, and eventually eaten by the Scarabs.