Well, gee, if you do, you're not in for good company.
Most people that kill the villagers do so because they don't like what the villagers offer them. If you don't have minecraft you don't know what I'm talking about. Well, you can trade things with villagers. Mostly its stuff like compasses. They normally want precious gems and such. However, these guys are some of the most stingy batches you've ever met. So, like most people, I dislike it when they give me poor offers. I kill them. Well, you know how it is, they just poof into thin air... this isn't MW3 or anything. Please don't get offended :P.
Anyway, if you kill villagers like me, you're about to be in for some consequences.
The head developer of Minecraft said this:
Used to kill villagers because you don't like what they offer? Well, in the future you will have to face the consequences of your actionsA link to his tweet.
So here's a new idea submitted by a Redditor.
Normal villagers should run away if you attack them and refuse to trade to trade with you. Then if you kill one/some, iron golems should target you and hostile villager knights will spawn. Villager knights move quickly at and have twice the health of a regular villager. They will start firimg arrows if you're within 32 blocks away and will suddenly run towards you at great speed if you're 8 blocks away to deal melee damage. Only way to become allies again is to wait some days until they've calmed down and then bring the villagers a cake. Fighting back will reset the time you need to wait before you can re-friendify them with cake. Knights will also protect you against most monsters if you're friend with the villagers, but you can't make them follow you, they stay inside the village's borders to defend it.
If you don't have Minecraft yet, what are you doing??? Get minecraft for free over here.