It's been a point of contention recently, the question of who created battle royale. Perhaps Greene is responsible for the modern iteration of it, but the last-man-standing concept has been around in gaming since Bomberman in 1983. And the concept of battle royale in general was popularized with the Hunger Games and of course the film named Battle Royale. And, "battle royale" as a phrase was used by Ralph Ellison in 1952 in his classic novel Invisible Man, describing a free-for-all boxing match that the protagonist must participate in. So Brendan Greene didn't come up with this out of thin air, or at least he wasn't the first.

On the other hand, I see no problem with him trying to profit from it either.

Edit: I correct myself -- in Invisible Man, the phrase was "battle royal," not "battle royale." It's still clearly similar however.