Ai in Civilization IV was a nice set of visible equations and with a bit of math you could get along quite nicely. Civ V introduced a new mechanic: random dickishness. Are you five turns in and getting denounced for "trying to win the game the same way"? Or maybe the previously friendly civilization has suddenly turned on you? Maybe you're on your march to victory when suddenly the entire world turns on you.
Sucks, doesn't it?
I think it's great. Let's face it, the world is not run by Kissinger clones who carefully balance practicality with bombing Cambodia for no apparent reason. Instead it is run by people. People get mad and paranoid. They lash out for nothing. They attack when they cannot win and surrender when they could win. They are irrational and random.
The AI system in Civilization captures this very nicely. Is it annoying to get invaded by your friend? Well sure. But you know who hated getting invaded by his best friend? Stalin. By the analogy this makes the AI Hitler, which is why it's okay to kill them.
In RPG terms, the AI lost its aggro table. You can't tank it easily with bribes and religion, though I do miss religion. Instead it attacks, not quite at random, but close enough that you'd better bring some disc priests, which I'm pretty sure means having the Great Wall and a lot of crossbowmen.