There are two scrapped features found in the game's files that would've occurred if you killed an NPC. The first was if you killed a unique NPC that has a family, then later you would see the ghost of that NPC follow any of their relatives, however only NPCs with generic voice files could do this, and only a single NPC in the whole game could turn into a ghost.

The second feature was after you killed a unique NPC who had a family, later talking to any of the NPC's family members will have them comment to you about their death in their hello conversation.