Got any NPCs that you enjoy putting an axe to?
For me, it's the guild leader of the baldur's gate thieves' guild.
He tells you that he has a job for you, planning to kill you when you are done.
If you insist on being paid upfront and knowing how much you are going to be paid, he attacks you.
If you do the job, he lets a powerful wizard attacks you, and only offers a half-hearted apology, explaining that he realizes now that it'd be too hard to kill you.
No reward. No real regret. No payment for a job that he sent you on. Just sadness that you are beyond his reach. There are few people as deserving of having the walls painted with their guts.
How about the rest of you? Any other NPCs that you enjoy taking down?