Gladiator isn't really a movie that lets its characters off the hook, but we think that this movie might be a little too kind to Quintus.
After all, he's clearly a very high-ranking officer. When we first meet him, he's Maximus' second in command. While he seems loyal to Maximus, as soon as Commodus takes over Quintus switches sides. He has Maximus arrested, at Commodus' orders, and the next we see of him, he's in charge of the praetorians, the Roman equivalent of the secret service. On orders from Commodus, he covers up the wound Commodus give Maximus shortly before the final fight scene.
Clearly, Quintus serves whoever's in power. He's a bit of a worm.
But we also can't really blame him. With a guy like Commodus running around, most of us would be scared to cross him. And to be fair, Quintus does kind of redeem himself. While he serves whoever is boss, he follows a higher code (sort of).
Once Maximus knocks Commodus' sword away, he asks Quintus to give him another one. Quintus doesn't say anything, and Commodus asks the praetorians for one. They all pull out their swords, but Quintus orders them sheathed.
Commodus has already cheated once, and he refuses to allow him to do so again. In the end, he helps carry Maximus' body out of the Colosseum, which suggests that maybe he finally gets it.