I actually think Oda planned Ace's destiny from the beginning. Not how he was going to die, or when he's going to die, but the simple idea that he will die.
Ace of Spades (card of death), the flip smilies hat (foreshadowing of a darker reality in Ace -> his self-hatred, etc etc), and the most defining sign for me is this: Ace is a good character with too much power and circumstances. Unlike many scrub characters who have no real "will" (Johnny and Yosaku for example), Ace had a strong will to make Whitebeard the pirate king. We all knew from the very beginning that Whitebeard will not be the pirate king - pirate king = Luffy. Something had to give. Making hatred spawn between Luffy and Ace would be bad for this style of manga no matter how you view it. The only other alternative was for something to go wrong for Ace. I've always thought that Whitebeard would betray his trust (the typical "my idol wasn't who I thought he was" routine), but when the character of WB was introduced, we knew right away that such a possibility may as well be non-existent. This leads to the only other way to terminate his will - by taking his life.
Anyway, point is, long story short, I don't think Oda hesitated with killing Ace - he knew all along what his role was going to be.
Edit: Wow my only serious post in a while 0.o