Battannia took nevyansk castle from my kingdom (sturgian culture) and when I took it back my only choice was to show mercy (since its my culture). What surprised me was the -10 relation hit I took with my spouse, Urik, who was a sturgian. My guess is it's due to him being cruel? If it is, I think this relation hit only makes sense for other cultures fiefs and not same culture fiefs since your only choice is to show mercy.