As with all such complicated subjects, there are many ways to divide it up.
The way you mention is one way. There are others.
Some realworld leaders are more trustworthy than others.
In the game, we would expect a leader named Gandhi to be much more trustworthy than a leader named Hitler.
Players have a right to build such expectations into their play.
(We are not saying Gandhi never attacks you and Hitler always does.)
CIV leaders already have a range of personalities and therefore sometimes act differently than each other in the exact same situation.
When I was younger I played a lot of the classic game Diplomacy.
Some players are more trustworthy than others.
All good players realized that backstabbing at every opportunity was not an optimal strategy.
All good players realized the importance of cooperation.
All good players realized that one should be less likely to attack someone who is and has been friendly towards you.
So if we were trying to simulate a good, mature player of Diplomacy, it would not be that much different than the current AI. (However, the player would be more clever, harder to trick, etc.)
1. Cooperate with some others.
2. Based on personality get a reputation for being either somewhat trustworthy or very trustworthy.
3. When and if you betray someone, make sure it is worth a lot.
4. Be less likely to attack those to whom you have a good attitude and more likely to attack those you dislike. More importantly, be less likely to attack those who like you and more likely to attack those who dislike you.
The problem is, there are two irreconcilable school of thoughts when it comes to diplomacy and the AI in general.
There is the 'Historical' one, which starts with the premise that the AI should behave as much like a real world leader would in the given circumstances. Obviously, as Civ is not a perfect historical simulation, there are limits to this approach.
The other school is the "Multiplayer" one, which wants the AI to be as close to a human player as much as possible. It means that it should be prepared, for example, to backstab you should you be about to win and never to vote for anyone else for a diplomatic victory.
In Civ3, the AI was closer to the multiplayer school, where it would gang up on you if you were about to win. The introduction of religious diplomacy in Civ4 made it much more historical, although the aggressive AI setting was meant to bring it slightly closer to the multiplayer role.
Personally, I'm still undecided between the two options, and depending on my mood and how competitive I feel I switch my preference from one to the other.
Which school of thought civ5 takes is still not 100% clear.
