Quintus was a disloyal idiot
He starts off the film as a seemingly loyal officer serving Maximus as the legion faces the German tribe, and follows his general out of the tent after the Emperor was slain, snubbing Commodus (if shyly). But soon follows the new emperor's orders to have his friend's wife and son murdered?
Then, not once does he speak up for, or even connive, to help Maximus one bit, except for the lame last fight whereby he orders (finally!!!) the praetorians to 'sheath their shields'.
Was he duplicitous, or a loyal career officer to the extreme like a mercenary?