No magic?
Forgot about that, especially when some players get skills that ressemble divination... or revelations about weapons for example.

Regardind the use of NPC, my guess is that using one a a tool to kill would still get you the kill. So use one that will use another NPC... I know, it's largely stretched. But I guess you could die from rocks falling on you, drowning and the likes. So potentially NPCs could accidentally kill you.
Now the details are how you control those accidents...