Hmm.  My character's not a big fan of bloodsport, so I'd probably imagine Adriaan was caught during a robbery gone wrong and was forced to join the arena.  If not killing your opponents is an option, though, he'd probably join because he really needs the money.  He wouldn't willingly put himself in a position where he has no choice but to kill, but he's fuzzier on the subject of kneecaps, and could talk himself into beatings if the price on his head was big enough.