This is one of the strangest fun facts from the movie. If you think of thumbs-down in modern terms, it’s pretty obvious for us that the emperor isn’t voting to save the life of the gladiator. In the same way, when the emperor makes the opposite gesture, today we understand that the gladiator should be forgiven. But actually, back in ancient times, it was exactly the other way around: thumbs up meant sword action, therefore the gladiator’s life would not be spared, and thumbs down, was meant to symbolize a sheathed sword, which would indicate mercy to the gladiator. The production team was fully aware of this, but since today thumbs up is considered to be something positive, they decided to not confuse the audience with historical accuracies.