During the botched execution about half an hour into the film, Maximus sustains a ghastly and disgusting wound. And Gladiator continues to focus on this wound for the next fifteen or twenty minutes. (Thanks a bunch, Gladiator.)
And this gash is pretty much the definition of hard to watch. It's full of maggots (ugh) and pretty weepy. In short, it's nasty business.
The wound, however, symbolizes a few things. Let's take a look at that whole "full of maggots" bit. Maggots are a) perhaps the nastiest creature on earth and b) pretty much always symbolic of an infestation.
Since Maximus' received his wound at the hands of would-be executioners taking orders from Commodus, the ghastly wound tells us that Maximus is now "infested" by Commodus. The desire to take revenge on Commodus has completely taken over Maximus' psyche…much as those wriggling lil' maggots (ew, ew, ew) have taken over his wound.
In a larger sense, however, the wound's also a symbol of Rome's betrayal of Maximus. After all, the wound is close to Maximus' SPQR tattoo. SPQR is an acronym that stands for Senātus Populusque Rōmānus, a Latin phrase that means "the senate and the roman people."
It's a clearly a sign that Maximus belongs in some way to Rome, and the fact that his wound is located next to it suggests that Maximus' very identity as a Roman soldier is in jeopardy.
It isn't in jeopardy for very long, though, because Maximus takes his knife and cuts out the tattoo… effectively giving himself yet another wound. (Doesn't he have enough?)
After Commodus kills Maximus' family, he wants no part of Rome. He symbolically exiles himself from the Empire he's spent his life serving. This is an absolutely necessary transition, for it symbolizes Maximus' transition from a member of the Roman citizenry and army to an outsider, a gladiator, and a slave.
Also—he ends up with some sick scars. That's always a bonus when you kill people for a living.