Joaquin Phoenix's Commodus is one of the great, thoroughly unlikeable movie villains: he's a snivelling wretch with an inflated and unearned sense of worth that basically ensures everyone wants him to get his comeuppance. And when he comes face-to-face with Russell Crowe's valiant hero Maximus, it looks like it can go only one way.
But then the monster does as everyone should have expected, fatally stabbing his opponent ahead of their duel and concealing the wound, to ensure his own victory. So while Maximus ultimately triumphs and rids Rome of its poisonous new leader, he has no time to celebrate, falling onto his back and dying in the arena almost immediately.
It's a bittersweet ending, for sure, but it's not exactly a downer. Despite the appeal of seeing Maximus live to find happiness in his freedom, his true journey is to return to his family, who were slain early in the film. So while watching him bleed out is a tragedy for those he leaves behind, there is happiness at his own end.