I would indeed be a delightful twist if Malfoy were to come to the side of justice. I think that it's entirely plausible because I've always had the feeling that Malfoy cares deeply for his family and friends if nothing else, and he very well may become disillusioned with the prospect of becoming a Death Eater if he things Voldemort will kill his family.
And about Harry being a Horcrux, here's just one of many quotes from the books that could back this presumtion up:
Page 815-816 of OotP said:
"And then Harry's scar burst open. He knew he was dead: it was bain beyond imagining, pain past endurance--
He was gone from the hall, he was locked in the coils of a creature with red eyes, so tightly bound that Harry did not know where is body ended and the creature's began. They were fused together, bound by pain, and there was no escape--
And when the creature spoke, it used Harry's mouth, so that in his agony he felt his jaw move. . . .
"Kill me now, Dumbledore. . . ."
Blinded and dying, every part of him screaming for release, Harry felt the creature use him again. . . .
If death is nothing, Dunbledore, kill the boy. . . ."
Let the pain stop, thought Harry.
Let him kill us. . . . End it. Dumbledore. . . . Death is nothing compared to this. . . ."
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I feel that the only way that this scene could have happened (since no Imperius Curse was uttered and Voldemort had already gone) is if Harry was a Horcrux. It was clever of Voldemort to take possession of Harry's Horcrux at this point in time--he thought at the cost of a mere seventh of his soul, he could have ended the life of what he believes is the only person who can rise to defeat him if he could only trick Dumbledore in that moment of terror into thinking that he had some necessity in killing Harry.
Though, as we all know, Dumbledore is not that foolish and he would have never killed Harry under the given circumstances. It does make me wonder whether Dumbledore knew of Voldemort's seven Horcruxes during this time--only a couple of weeks after this incident does Harry see Dumbledore with his blackened and withered arm, so I'm leaning toward Dumbledore knowing of them but believing that the quoted incident occured because of a non-Horcrux related link between Harry and Voldemort. My point being, it makes me wonder if Dumbledore would have spared Harry's life in that moment if he had had the realization that Harry contained a Horcrux.