I am playing as Dunland and I wanted to ally myself with as many Good factions as possible with the exception of Rohan, starting with Durin´s Folk because they´re friends.
I made sure I have negative allignment with all evil factions except for Dunland, I gained allignment with Durin´s Folk and then went to kill Rohirrim. When I started killing them, Durin´s Folk allignment started draining.
I thought it was because, even though I have negative allignment with their mortal enemies, I am still gaining some points for killing Rohirrim. So I went all the way to the edge of the sphere of infulence of both Isengard and Mordor and killed Rohirrim over there. There, I recieved no points for any evil faction except Dunland, despite that my Durin´s Folks allignment began draining upon killing Rohirrim.
Rohan isn´t even mentioned on the Durin´s Folks relation page. Why is it causing an allignment drain with them when I kill Rohirrim ?
Further, I am not sure what exactly causes allignment drain. Is it merely the action of recieving any points for the mortal enemy faction ? Or is it supposed to happen only once you have a positive allignment with the mortal enemy faction ?
In my case, it happened upon me simply killing Rohirrim, even though I wasn´t recieving any poitns towards Drun´s Folks enemy factions.