Commodus is shown in Gladiator as arriving on the battlefield after the action is complete. In fact, he campaigned with his father and participated in battles from the time he was five years old and actually was in command of the troops in the successful campaign shown at the start of the movie. Maximus, the movie's heroic and victorious general, never existed, and his fictional victories really belonged to Commodus. Three years before his death, Marcus Aurelius named Commodus co-Emperor in recognition of his military victories, but Marcus Aurelius actually ruled alone until his death. That occurred in 180 AD, of disease usually identified as "the plague" (medical authorities differ on what that might have been), somewhere near modern Vienna (and historians, of course, argue about exactly where). A few enemies of Commodus later mentioned poison, but there's really no evidence (and it was always "poison" and never smothering that came up in such cases.) Lucilla wasn't in the military theater.
Nobody ever was able to determine the exact fatherhood of the many children produced by Faustina the Younger, the wife of Marcus Aurelius, but both Lucilla and Commodus were among her brood. Lucilla was older by a number of years, and she really had married Lucius Verus when Commodus was four years old. Verus died ten years before the action of the film starts, so it is unlikely that there would be an eight-year-old son of theirs running around several years into the epic. Lucilla did hate Commodus and she participated in an early assassination plot for which she was exiled and then murdered on orders of Commodus. There was never any hint that he took her to bed, but there was plenty of evidence of such activity with some of his younger sisters -- nobody knows how willing they were. (Lucilla, who had remarried, was also promiscuous -- just not with her brother.) Commodus' main squeeze was a cousin named Marcia, and there will be more about her later.
Shortly after Marcus Aurelius' death, Commodus signed treaties with the Germans and withdrew all Roman forces from north of the Danube River. This move was immensely popular with the Army, who had been campaigning there relentlessly for almost 20 years, and with most Romans, who were tired of the high casualty rate and of the economic drain caused by war profiteering by many Senatorial families. Commodus brought the 180,000 man Danube Army home to an immense triumphal celebration, which he dedicated to his deceased father. But it was Commodus who was the people's hero. His popularity was bolstered by the unprecedented long series of gladiatorial games associated with the Triumph and with large distributions of money to all Romans.
Commodus sponsored additional games throughout his reign and always gave out cash to help people enjoy them. He was arguably the most popular Emperor that Rome had since Augustus. Of course, he was hated by the upper classes -- it was their war profits he was distributing to the masses. Modern economists have estimated that his expenditures for "bread and circuses" were huge, but that he spent much less than the "good emperors" before him (and especially Marcus Aurelius) had spent on border wars that were of marginal to negative value to the already over-extended Empire.
Commodus did fight in the arena, but he had an understandably perfect record. His gladiatorial opponents knew that, if they put on a good but not dangerous show, they would only receive a minor wound and would be spared in the end by the merciful Emperor. He was also an accomplished animal slayer and had lions, bears, leopards, hippos, and a giraffe on his lists of conquests. Some of these he slew from safe platforms and catwalks, but he also sometimes went down to the arena floor. One of his great crowd pleasers was shooting the heads off running ostriches with specially designed broad-headed arrows -- they'd run around headless for a while, much to the delight of the crowd. And it should be remembered that none of his excesses put off the audience. Bloody as the movie is, it cannot compare with the hellish scenes played out in the real Colosseum -- scenes relished by the Roman public -- men women and children alike. Some revisionists have gone so far as to say that Commodus just played to his audience and that, perhaps, he wasn't as mad as he seemed. (This is the Hamlet debate a millennium and a half sooner.)
I think he was really nuts. And really evil. Saying that he was just an ultimate expression of his society doesn't excuse. By the end of his rule, he had apparently identified himself with both Jupiter and Hercules. He took to wearing a Herculean lion skin and keeping a Hercules style club next to his throne. His arena persona took over his life, and he turned civil administration over to lackeys and freedmen. With his encouragement, they increased the number of executions and property seizures among the upper classes, always trying to get enough money to mollify the Roman mob. Commodus eventually tried to rename the months of the year after his own names and titles, and he proclaimed a new name for Rome: the "Colony of Commodus". Ultimately his extreme behaviors and megalomaniacal beliefs became habitual, and because of them he ended up with many more powerful enemies than he could defend against.
The Gladiator film vastly telescopes the reign of Commodus, showing him being killed early in his reign in the Colosseum. He really lasted thirteen years, and, at the end of 192, he was poised to seize the last remaining public offices for himself. On New Years day 193 he planned to go before the Senate (dressed as a gladiator, with the Hercules lion skin on his shoulders) to take over the offices of the two elected Consuls after having them murdered. The story then becomes murky.
Perhaps because of "information" supplied by enemy Senators, Marcia (Commodus' mistress/cousin), connived with some of Commodus' cronies and the wrestlers and gladiators he hung out with, and they convinced each other that they were all on Commodus' latest hit list. The story goes that they first poisoned Commodus, but the dose was only strong enough to render him unconscious. Fearing that he might wake up in a mean mood, Marcia persuaded one of the wrestlers to strangle him. Commodus never saw the dawn of the new year.
The Senate rejoiced and proclaimed famously that Commodus' corpse should be dragged through the streets with "the hook" -- that was how really badly disgraced dead gladiators were taken from the arena. He was saved from that post-mortem indignity by one of the high-ranking plotters, who spirited the body away to an unmarked grave outside Rome.
The death of Commodus marked the beginning of a new series of civil wars. His immediate successor, Pertinax, lasted only 87 days before the army killed him off. Then Didius Julianus offered the Praetorian Guard the biggest bribe, and they made him emperor. When he couldn't deliver the bribe money, he soon lost Praetorian support. Provincial armies proclaimed their own emperors, and, when Septimius Severus marched on the city, Julianus was killed an officer in his own guard platoon -- only 66 days after the Senate had proclaimed him. The Senate then proclaimed Septimius Severus Emperor, but it took him four years to root out other claimants. The history of the end of Commodus and the fate of the "Severan Dynasty" is in one of the Internet links below.
Because of the popularity of Gladiator, Commodus is suddenly one of the hottest items on the Internet. Some of the better and more interesting sites are:
The Severan Dynasty link, which starts with a history of Commodus: http://www.ancientsites.com/er/severans.html (includes the famous Senate "hook" proclamation.)
Biography from "De Imperitoribus Romanis": http://www.roman-emperors.org/commod.htm
Edward Gibbon's take on Commodus from The Decline and Fall·, Etc.: http://members.aol.com/heliogabby/private/commogib.htm .
Commentaries on the Gladiator movie and comparisons of the movie with history. (You should remember that Commodus' enemies got to pay the next generation of historians, whose works became the basis for "historical fact".)
From a respected Canadian columnist, Robert Fulford: http://www.robertfulford.com/MarcusAurelius.html.
From "Gladiator, the Real Story" web site: http://www.exovedate.com/the_real_gladiator_one.html
"Who was Commodus" web site: http://moviegladiator.tripod.com/wwcom.html.
Buy your own 19th century bronze statue of Commodus dressed as Hercules, copied from an ancient Roman statue in marble. Perfect for the mantle -- 25 inches (63 centimeters) and only 2950 UK pounds ($4,412): http://www.antiques-in-england.com/hercules.htm.