How many of us have played a game like GTA and gone on the rampage, smurfing up everyone's trout for no other reason than fun? This is just the same principle with more advanced technology.
To the best of my knowledge the NPCs I killed were not sentient nor remember what I did so I don't feel any guilt over it. Sure, they ran away screaming and cried out in pain when I shot them, but that was just their programming. It's part of making a realistic experience. If I was suddenly made aware that actually they have the same cognition as the hosts in Westworld I would feel like an almighty piece of trout and wouldn't do it again. But they don't... I hope