In the new 1.9 update, NPC were included. Do they have any AI, or are they just like passive animal mobs? What will happen to them when their houses are destroyed, if anything?
Right now they are passive mobs, walking around randomly and watching you from time to time like cows and pigs. When you destroy their houses, nothing will happen. I destroyed an entire village and the villagers where still there after hours.
From the Minecraft Wiki:
Villagers have the AI of a pig. This is probably a placeholder.
This shows how they interact with the player and that their programming probably defines that they won't despawn if you destroy the houses.
I read somewhere (can't find the source right now) that maybe the NPCs will be a community in a later update. If you kill one, all of them will turn hostile. I would like that idea, because right now (pre-release 1.9) they look kind of stupid. (Especially since the have male genitals in their faces.)