According to the movie's screenplay, the character of Commodus is based on historical facts, while not exact, but intended to mirror the personality traits of Emperor Galus Germanicus, who has become infamously remembered in history as "little boots", or Caligula.
Similar to Caligula, Commodus seems to have pervasive mental illnesses that render them psychologically limited, creating episodes of histrionic personality,narcissism, megalomania and psychopathy. These traits are often historically conceded as genetic complications due to possible inbreeding among generations.
However, in a dramatic difference from Caligula, Commodus was not the beloved son of his father, nor was he expected one day to succeed his father's great feats and become Emperor as well. His asinine ways led his father to bypass the bloodline and make Maximus, a soldier, as Emperor. What this did was exacerbate Commodus trouble mind, and make him literally more insane.
Since Commodus does not have any needs, his life is devoted to sick wants. As a result, Commodus usurped the throne and engaged in terrible practices that fed these wants over and over.
In the screenplay, Commodus is supposed to have entered his new life as an emperor after having nearly destroyed Maximus's life. His first want was lust and that is how he began to reign.
the emperor Commodus spent the early years of his reign "in a seraglio of three hundred beautiful women and as many boys, of every rank and of every province."
The second deadly sin of Commudus, which became the main ambition of his life, was blood- he set out to conduct wild games in the Colosseum, and to add murder as one of his hobbies.
Later, adding bloodshed to his round of pleasures, he launched a career in murder, beginning with the dispatch of the usual senators, ministers and family members and continuing with the slaughter of beasts.
Finally, since his megalomania knew no limits, Commodus based his ambition of mere gluttony for power, for he needed no more than what he already had.
Styling himself the Roman Hercules, he went as a performer into the amphitheater, where he cut down before the public a number of ostriches, a panther, a hundred lions, an elephant, a rhinoceros and a giraffe. He then entered the lists as a gladiator.
As a gladiator, Commodus paid himself enormous fees that he also did not need. In all, Commodus is merely a sick, spoiled man with too much power and no more ambition than to proof himself worthy of something great. Unfortunately that great deed is to outdo his own mischief and make himself stand out by pushing down who is the true pride and joy of Commodus's father, the usurped emperor Maximus.