"One of the reasons we are attracted to the Colosseum is because of the incredible violence that went on here. The question it poses is, how could such an advanced culture have staged such bloody spectacles? The Colosseum is a snapshot in stone, a physical embodiment of the culture of Rome."

-- Gary Glassman (quoted in Colosseum killing machine reconstructed after more than 1,500 years

Emphasis mine: I'm not surprised. Culture is relative and I see lots of reasons this could have been beneficial to the society at that time.