I think Nic would prefer not to kill Harry. (He'd much prefer Harry as a Denarian.) Nic probably knows about Starborn, and I don't think he wants to see reality destroyed by the Outsiders.
Some of it is the increasing scope of the series, as well. Back in book 5 when Harry was "just" a powerful-but-fairly-inexperienced wizard dealing with sorcerers, vampires, and Nevernever "thug" demons, the Denarians were terrifyingly powerful. By book 15, Harry the Winter Knight with Soulfire is far more powerful, and has been involved in huge-scale supernatural battles in Changes and CD.