Yes...but to an extent. Sora could go on a rampage and start working for darkness, but I think the Keyblade would go find another wielder. But then again, Riku when he was in his right mind, stole the Keyblade from Sora... Nevertheless, The Soul Eater that Riku had while possessed by Ansem, that is a prime example of Wielder choice. Riku used it to reveal the Keyhole and a portal to darkness, while Sora used it to restore the hearts to the Princesses. Now, certain Keyblades are different. I think that the ones that take the place of grass in Sunset Horizons are just metal sticks. Good for beating people to a bloody pulp. Anyone can use them. But, like I said before, Sora's cant be used that way. Way to the Dawn is complicated. It is both Light and Dark, and is used by a redeemed person. So, if he went evil, who knows, he might still be able to use it. But ultimately, yes, Wielders decide.