Huge Warning for NPC Killers

[Utopos]

Started my NG+, decided to take on the Four Kings first. Killed Ingward for the Seal Key only he didn't drop it. Quit and reloaded my save, nothing to pickup still.

Went ahead and killed every boss up to getting the Lord Vessel. Still no key to be seen. Can't be purchased from Domhnall under the bridge in Firelink. It's not in the box behind Frampt. This play-through is now ruined utterly but no big deal.

Fair warning, don't go killing Ingward on NG+ or you'll be stuck.