She said she wants the fruit to kill Donflamingo. They already said that this particular event was voluntary. But most of them aren't. Remember the whole "survive 1000 matches and you go free" thing they talked about before?
But the simple fact is, if this is the fighting style she's been using in the colosseum for the last decade or whatever, and she's never even been hit before, it would be stupid of her to try and do a bunch of stuff she's never done before now.
No, the flashbacks made it pretty clear. Her reason is because her mother told her she shouldn't hurt people, but Soldier said she needed to become strong to protect herself. This is how she's doing both. She's a completely defensive fighter.