Outland

In the future, Federal Marshal William O'Niel (Sean Connery) is assigned to a tour of duty at the titanium ore mining outpost Con-Am 27, operated by the company Conglomerates Amalgamated on the Jovian moon of Io. Conditions on Io are difficult: gravity is 1/6 that of Earth's with no breathable atmosphere, spacesuits are cumbersome, and miners carry their own air supply. Shifts are long, but significant bonuses are paid. The outpost's general manager, Mark Sheppard (Peter Boyle), boasts that productivity has broken all previous records since he took over.
O'Niel's wife Carol (Kika Markham) feels she cannot raise their son Paul on Io, fleeing with the child to the Jupiter space station to await a shuttle back to Earth. Later, a miner named Tarlow (John Ratzenberger) suffers an attack of stimulant psychosis: he sees spiders and rips open his spacesuit, resulting in his death by explosive decompression. Cane, another miner, enters an elevator without his spacesuit during another psychotic episode, and likewise dies from decompression. With the reluctant assistance of Dr. Lazarus (Frances Sternhagen), O'Niel investigates the deaths.
Another incident involves a worker, Sagan, who takes a prostitute hostage and threatens to kill her with a knife. O'Niel attempts to calm the man while Montone (James B. Sikking), his sergeant, sneaks in via the air duct and kills Sagan with a shotgun. O'Niel and Lazarus discover that Sagan had traces of a powerful amphetamine-type drug in his bloodstream, which would allow the miners to work continuously for days at a time, until they "burn out" and turn psychotic after approximately ten months of use. O'Niel uncovers a drug distribution ring run by a corrupt Sheppard and sanctioned by Montone.
Using surveillance cameras, O'Niel finds and captures one of Sheppard's dealers, Nicholas Spota. However, before Spota can be questioned, he is murdered. Montone is then found garroted. O'Niel finds the latest shipment of drugs in a meat locker that was shipped from the space station, but is then attacked by another dealer, Russell Yario. O'Niel knocks him out, then destroys the shipment of drugs. When Sheppard finds out, he threatens O'Niel and contacts his drug distributor, asking him to send in professional hitmen. O'Niel is ready, though, having been monitoring Sheppard's communications.
O'Niel waits for the arrival of the hitmen on a supply shuttle from the other side of Jupiter. Knowing what's coming, and with only Dr. Lazarus willing to help him, O'Niel sends a message to his family promising to return to Earth when his "job is done". When the hitmen arrive, O'Niel tracks and ambushes each, killing them one by one. Lazarus helps him kill the first by trapping him in a pressurized corridor; O'Niel activates a bomb, causing an explosive decompression that kills the hitman. The second is killed in a greenhouse structure when O'Niel tricks him into shooting a window, causing it to break open and blow him out to his death.
O'Niel is then confronted by Sheppard's "inside man", who turns out to be one of his own deputies, Sgt. Ballard. The two fight outside the outpost on a solar panel structure until O'Niel pulls Ballard's oxygen hose, suffocating him. O'Niel then confronts the surprised Sheppard inside the outpost's recreation bar, knocking him out with one punch. It is implied that Sheppard will now either be murdered by his own accomplices or brought to justice, while O'Niel's deputies will be brought to trial for mutiny. O'Niel retires, parts ways with Dr. Lazarus, and leaves on the shuttle to join his wife and son on the journey back to Earth.

Question: Who turns out to be Sheppard's inside man?
Answer: Sgt. Ballard