I'm really uncertain how this managed to make it through this far given the extensive testing done 'versus the ai'. But Kraxus has a faction ability that is designed around the use of razing cities. It's function is dependant upon the population of said razed city... which means for all practical purposes this ability will never see any use. As an aside it's a pretty terrible ability given how quickly the capital maxes out.

On top of this there should be a solid reason to attack other players (risk vs reward) rather than simply to occupy more land. On the flip side of things, it is unreasonably harsh to reduce the population of a conquered city to zero if the defenders ever plan to retake it. This 'might' make sense for the fallen vs the empire since they are different races, but even then a single turn to kill hundreds of people seems unreasonable.