Since I began working for the Thieves' Guild, I've noticed the unnamed thief NPCs that show up in towns every so often to cause a bit of mischief.
Unfortunately for them, they aren't very adept, and tend to die rather quickly to the town guards' arrows - I don't really mind this, as they often carry shiny things that aren't marked as stolen and no one minds me taking them.
If I were to hasten their demise by accidentally brutally stabbing them to death, would anyone mind? Will the Guild make any reprisals? Is it better to just wait for them to blunder into a guard?