Norman Jewison was apparently inspired to make Rollerball originally when he was watching a hockey game, and a fight broke out. The crowd was so excited about the fight (as opposed to the game) that Jewison was quite shocked...and decided to take the idea to an ultimate end. Not so much the death games of the Colosseum, but instead actual sport with extremely violent results, supported by the fans.
As an interesting side note, the movie adaptation of "The Running Man" (with Arnold Schwarzenegger) seemed to also take on the theme of corporate games (though with more emphasis on death) to appease the bloodlust of the public, although the movie itself is very different from the original Richard Bachman tale. It's uncertain if any inspiration was derived from Rollerball for that film.