I think a blend of Morrowind's essential character system and Oblivion's essential character system would be best. A character could be marked as essential, making them unconscious at the hands of everything (including falls, enemies, rocks, traps, etc) except the player. This way the player could kill essential characters, not have to wory about other non-player characters killing the essential character, and let the character's environment weaken the essential character before killing the essential character should the player choose. When an essential character has been killed, the player could a message much like the one in Morrowind.

It would be just as easy to make a "replacement" NPC that does the same service or gives you the needed Info.