As it was my example, I should perhaps clarify some context.That's ... not what the example was. The PC's kill the emperor and the DM decides that a civil war occurs. I'm just using the example given.
The PCs' original mission in that realm was to, if they could, knock off the evil Emperor; widely (and correctly) assumed to be a lich. Some of the same party had recently finished a version of the Slavers series and had met the various slave lords while largely busting up their slaving operation, so now there's a bunch of deposed and scattered slave lords looking for trouble.
Party goes in, and against some rather long odds does in fact manage to knock off the lich Emperor. Thinking it a job well done, they go home and then carry on to other adventures elsewhere, completely ignoring what they left behind them. Meanwhile, the deposed slave lords (each of whom already has a power base in or near this nasty land) plus another powerful NPC, all see an opportunity: there's an empire waiting to be taken. Negotiations between these parties weren't going to go anywhere to start with, thus civil war would erupt unless interrupted. It wasn't interrupted. A 5-way civil war followed.
How did I determine all this? A combination of random rolls plus knowledge of the personalities of the various leaders (mostly ex-slave lords).
The next time any PCs (only a few of whom were in any of the Slavers series) visited that land the war was well underway, providing a quite different backdrop from when anyone was there before. Since then assorted PCs have fought in one of the armies, spent some time as welcome guests in the camp of another, knocked off the leader of a third (not that it helped any), and had peaceful dealings with soldiers of several. A party on an unrelated mission following up on Slavers, a year or so later, also unintentionally busted up a peace conference where the five leaders were trying to come to some sort of agreement - oops - leading to the murder of one of the army leaders shortly followed by the disbanding of her army; so now it's a 4-way war.
Since then that war has largely faded into the background, though it's still ongoing and snippets of news occasionally arrive elsewhere; and play has moved fairly far away from that land.