Plus, I don't like Ace. If he died, I would finally be relieved that he wasn't as strong as Luffy. Or... something... Anyway, I just don't like anyone being stronger than Luffy. He can kick anyone's ass at this point. Period.
Besides, Ace gave Luffy that slip of paper. I bet Oda forgot about it, and he'll remember about it a few days after he kills Ace. HAHAH.