In Civ V, that was introduced in one of the last two patches.
The base warmonger is :
DOWing anybody (including city states) or Taking someone's last city (including city state) (each): Max penalty (each instance about 50% towards the AIs that care most about warmongling will hate you forever).
Conquering a city but they still have at least one remaining: max penalty / [total number of cities they previously owned] (Second to last city is 50% as bad as taking last one, taking 3rd to last one is 1/3rd as bad as taking the last one, etc.) : Cumulative ; introduced either in one of the last vanilla patches before G&K or with G&K.
What the BNW patch introducing era modifier did is add additional modifiers if the game hasn't yet reached Industrial era. I think Ancient & Clasical Era are 50% of above penalty and Midevil / Rean era 75% of above penalty. The numbers are given in whichever patch notes introduced it.
Notes:
1. Dead men tell no tells in Civ V (if you wipe out the civ prior to anyone else meeting you it doesn't count.)
2. Several AIs also have flavors that strongly don't like those attacking city states : And so they'll dislike those that do and it's independent of warmonger. (Others are moderate and still others won't care) ; these exact same ones become cracky and those that bully city states.
3. While some AIs really hate warmonger other's are "moderate" and a few are "do what you like"
4. The Civ you attacked / took cities from has additional modifiers to hate you
5. If another AI is at war with the civ you took cities from then with a fully patched game they aren't supposed to care. In addition if they aren't at war but have denounced them it's supposed to care less than if it hasn't.