Oda doesn't like killing villains, and it's not because he is 100 percent opposed to killing. But it's because he believes they should have their dreams crushed and live with that knowledge. For some it is a fate far worse than death.
But even if Oda does decide to break that mold (I know that was how he felt years ago, not sure if he still does), killing Orochi here doesn't make a whole lot of sense. He'll never have to face the Scabbards for what he did. He'll never be confronted, nor have to own up to what he did. Dying now, is the easy way out. Toki's prophecy amounts to nothing. Straight D&D from Game of Thrones fuckery, in all honesty.