That's probably because it's difficult to balance those two goals. I do it as a human, but not successfully every time, and I have to take a long, long-term view when my economy is hitting the skids in the middle of a long war of conquest. I'm not sure if the AI is (or even can be) programmed to set and stick to long-term goals; my understanding is that the AI makes a new set of decisions every turn.
Well, warmongers seem to be defined by three things in Civ IV: they usually have poor tech, they usually have lots of units, and they always attack frequently. You could switch the first two (good tech but few units) and if they attack frequently or with little provocation, they'll still be a warmonger.
Maybe someone could tweak Darius's peaceweight or something and see what happens.
My list of pushover AI (for various reasons):
Isabella: just take her religion if you're on the same continent, or free religion if you're not and watch the other religions beat the snot out of her
Mansa Musa: everybody ALWAYS hates him (I don't know why – could be worst enemy tech trades), and his UnitProb is 25, which is the second lowest tier (unless you count Gandhi's 15), so he gets steamrolled despite his tech
Churchill: sucks fighting him, but he doesn't seem to do too well in general except for during the reign of the redcoats
Tokugawa: he's a total isolationist, so he almost always sucks. Backwards tech, poor cities and commerce, usually just sits big stacks in his cities. Pretty annoying, but not much of a threat (usually)
Sitting Bull: got treated almost as badly in Civ as he did in real life, but arg the spies!
Gandhi: find him and kill him, or bribe someone into killing him. 15 unit prob and nice, juicy cities. Sometimes he has powerful friends though, or more likely a powerful master
Genghis Khan: VERY risky, but whereas some warmongers get good early starts by capturing some nearby enemies, Genghis has a 75% chance to raze captured cities, so unless he starts right by you, even if he has a powerful military, he won't actually start gaining from his exploits until Feudalism. Seems that his power is directly proportional to the map size