Joaquin Phoenix's Emperor Commodus murdered many people to secure his position in Gladiator, but no murder was more heinous than the killing of Maximus' wife and young son. Phoenix's terrifying yet fictionalized portrayal of the Roman emperor was founded in the character's cruelty, but the most intimidating aspect of the character is how calculating he was in his quest to gain and retain power. In that light, his murder of Maximus' family is much more than just a random act of violence meant to hurt a perceived enemy.
Commodus' path to power in Gladiator is littered with bodies, beginning with his own father and continuing through his foiling of Maximus' grand escape plan to rejoin his army and seize control of Rome. While it certainly seemed that his father's murder was a crime of passion, given that up until that point Joaquin Phoenix's Gladiator character expected to be named Marcus Aurelius' successor, every murder after that carried some sort of purpose. That is particularly true when it comes to his murder of Maximus' family, which came after Maximus' death and seemed, at first glance, to be unnecessary.
Commodus Killing Maximus' Family Sent A Message
Murdering Maximus' family after Maximus himself was already dead (or so Commodus thought) seems like overkill, but in doing so, Commodus sent a message to all those who might consider questioning his rule. Seeing that even one of Rome's most beloved leaders can pay the ultimate price for disloyalty sends a message, but killing his family too changes the message. Maximus' murder acts as a warning, but his family's murder can shock the soldiers and Rome's other generals and senators into line. Commodus attempts to intimidate the senate to their faces later in Gladiator, so sending a message in this way is certainly not out of character.
The popularity of Maximus unfortunately further justified his family's murder for Commodus at the beginning of Gladiator. Given that Maximus seems to have personal relationships with many of his own officers and even foot soldiers, it seems likely that news of what happened to Maximus and his family would spread through the ranks quickly. As cold and calculating as it is, the murder of Maximus' family was the best way for Commodus to send an immediate and lasting message about the price of disloyalty.
Why Commodus Was Right To Worry About Maximus' Influence
As terrible as his response was, there is plenty of evidence to suggest that Commodus was right to be concerned about Maximus' influence. In the famous opening scene of Gladiator, Maximus demonstrates just how pervasive the respect for him is in his army, exchanging greetings with everyone from fellow officers to infantry soldiers. He is saluted at every turn, and as if that wasn't enough, it's revealed that Marcus Aurelius trusts him more than his own son. With influence at every level of the military, there is no person more dangerous to Commodus' rule than Maximus.
The ultimate example comes towards the end of Gladiator, when Maximus is able to work with Cicero, Senator Gracchus and Commodus' real-life sister Lucilla to orchestrate a coup. While the plan is snuffed out by Commodus before it gets off the ground, Cicero made it clear that thousands of Maximus' soldiers would answer his commands as soon as they found out he was alive. That kind of influence, born of love and respect, can not be bought or matched with fear. For Commodus, that was more than enough to justify the murder of Maximus' entire family.