It was changed from instakill in rP12792 (female citizens) and rP13400 (cavalry and infantry) reason from IRC (for the female citizens at least):
2012-12-02: 20:24 < Mythos_Ruler> Just messing with some stuff. I'd actually prefer slaughter to not be 1000. A couple of strikes would be nice. 2012-12-02: 20:26 < Mythos_Ruler> I was just playing with stats and accidentally committed. ;)