>They are evil. We kill evil! Let's gather and kill evils.

But only evils. To protect those whom the evils would otherwise harm. The drow mage you mentioned in the post wouldn't have any remorse for using an elf child for target practice with his offensive spells or as a guinea pig for an experimental spell. The Marans will hunt that mage so that the elven child would have a chance to grow up without being killed or otherwise harmed by the evil mage. Marans acknowledge that their path is a bloody one, and therefore it is important for them to always remember why they are killing. It is considered to be a worst thing that can happen to a Maran if he becomes like those whom they slay, which is a real concern for such a bloodthirsty group. This part of Maran RP doesn't convey to evils, whom they are supposed to be killing anyway.

Empire, on the other hand, fights others to subdue them to increase its own power, using all means to achieve this end.

The big question is why they are killing, not just both kill. That's where the difference lies. Unlike Empire, Fortress doesn't fight to grow more powerful.