Dark Souls Remastered | Killing friendly NPCs - Ingward in New Londo Ruins

If you haven't yet acquired the Lordvessel, Ingward will not provide the Key to the Seal, which is necessary to access the lower part of New Londo Ruins and take on the Four Kings. If you kill Ingward, he will drop the key and you can reach this area and defeat the boss before ever acquiring the Lordvessel.

Keep in mind you still have to kill Sif (Sif, the Great Grey Wolf) in order to obtain the Covenant of Artorias ring, which is the only way to traverse the abyss and reach the Four Kings without dying.

If you interact with him after the Four Kings (as I do here), he provides a different dialogue and no longer sells you anything anyway.

Killing him awarded me 1000 souls and no loot (on NG).