algorithm for murderous interactions?

[BerniniCaCO3]

I like to prune my village from time to time, and sometimes the possible interactions list makes no sense.

The king can send most people to the pit, but not always just anyone (of course quest-necessary characters are excluded, I'm not talking about them).
The spy can only challenge some people to duels and duels to the death; I haven't found rhyme or reason to which characters!  Other heroes, yes; and heroes' spouses, it seems; other town folk it's just hit or miss.

The same goes for poisoning.

In particular, I want a new king's counsel.  He can't be sent to the pit, it doesn't seem, and when playing the spy I can poison his food (which reaper's scythe does not, in fact, have an effect with) but am unable, ever, in several attempts, to poison his drink.

How do I get rid of him?

And, just what ARE the rules governing which characters you can assassinate or kill in duel, and which you cannot?

[MarianT]

A knight can challenge the monarch to a duel to the death -- I did that once.

[Audren]

If the person you wish to duel has a reason to be able to duel, you should be able to duel them. People you can duel include the Monarch, Knight, and Spy heroes, the Executioner, the Guards around the town, the Constable, Bandits if your kingdom's security is not high enough, and special characters in certain quests. Some of these cannot be dueled to the death, however. (I believe the Constable and Executioner.) I can't say anything about poison, as I have not done that. As for the counsel, I do not believe you can get rid of him, but I may be wrong.

[Seabody]

By counsel, you mean advisor, right?
I've killed him before. It was a while ago, and I've forgotten how I did it, but it can be done, I believe.