Gladiator

In AD 180, Hispano-Roman General Maximus Decimus Meridius leads the Roman army to a decisive victory against the Germanic tribes near Vindobona on the northern frontier. Now weary of battle, Maximus only desires to retire to his Spanish farm estate. But Emperor Marcus Aurelius tells him that his own son and heir, Commodus, is unfit to rule and appoints Maximus as regent to help save Rome from corruption. When the emperor reveals his plan to his son, Commodus murders him in a fit of rage.
Commodus announces he is the new Emperor and asks Maximus for his loyalty. When the general refuses, he is arrested by his officers and is sentenced to death at dawn. He kills his captors and rides for his farm. However, he arrives too late to find it destroyed and his family murdered, under Commodus' orders. Maximus buries them and collapses from his wounds and grief. He is found by slavers who take him to Zucchabar, a colonia in the Roman North African province of Mauretania Caesariensis, where he is sold to a lanista (gladiator trainer) named Proximo.
Although reluctant at first, Maximus fights in local tournaments and makes friends with two other gladiators named Juba, a Numidian, and Hagan, a German. As he wins every match because of his military skills and indifference to death, he gains fame and recognition. But Proximo, who reveals that he himself was once a gladiator who fought well enough to be given his freedom, advises Maximus that being a good killer is not enough; a good gladiator is one who can "win the crowd". Proximo encourages Maximus to go to Rome and fight in the Colosseum because Commodus has organized 150 days of games. He could then use the power he commands in the arena as leverage against the Emperor.
Maximus' first gladiatorial combat in the Colosseum is a re-enactment of the Roman victory over Carthage at the Battle of Zama. Although the gladiators (portraying the Carthaginians) are expected to be massacred, Maximus leads them to victory over the legionaries of Scipio Africanus. This prompts a surprised and delighted Commodus to enter the arena to offer his personal congratulations. When the emperor's young nephew, Lucius, also joins them, Maximus decides not to kill Commodus. Instead he reveals himself and vows to have vengeance. The Praetorian Guard is ordered to attack but this angers the crowd. Under pressure to keep the mob of Rome happy, Commodus angrily relents.
Maximus' next fight is a victory against a large undefeated gladiator. Although the crowd wants a kill, Maximus spares his defeated opponent's life. This defiance earns him the moniker "Maximus the Merciful" and more cheers of adulation. Angered at this outcome, Commodus enters the arena to taunt Maximus about his family's death. But the gladiator turns his back and walks away, another defiant act that is making him more popular than the Emperor.
Maximus discovers from Cicero, his ex-orderly, that his former legions remain loyal. Lucilla, Commodus' sister, and Gracchus, from the Senate, meet secretly with Maximus. He obtains their promise to help him escape Rome, rejoin his soldiers, topple Commodus by force, and hand power back to the Roman Senate. However, Commodus learns of the plot from Lucilla by threatening Lucius. The Praetorian Guard arrest Gracchus while others are sent to the gladiators' barracks. Maximus escapes after Proximo and his men (including Hagen) sacrifice themselves. But at the rendezvous point, Maximus is captured and Cicero is killed.
Commodus challenges Maximus to a duel in the Colosseum. However, he secretly stabs Maximus in the side to gain an advantage. Nevertheless Maximus eventually disarms Commodus. When his Praetorian Guards refuse to give him another sword, the emperor produces a hidden knife, but Maximus drives the blade back into Commodus' throat.
As Maximus lies dying, his last words are to ask for political reforms, his gladiator allies freed, and for Senator Gracchus to be reinstated. While the body of Commodus is left unceremoniously on the floor of the arena, Maximus is solemnly carried away to be given an honorable funeral as a "soldier of Rome". A now-free Juba revisits the Colosseum at night. He buries the figurines of Maximus' wife and son at the spot where he died. He says he is now going back to his own family but promises to see Maximus again, "but not yet".

Question: Who murders the emperor?
Answer: Commodus