There are several examples in Fort dogma that draw a line between good and neutral. The whole purpose of the Fortress is to protect the "Light", which refers to good-align. It does not extend to neutral. Also, things like the Acolyte power to detect align via tells only differentiates between "good" and "not good".
In other words, there is nothing in Fort dogma that suggest a Fort member (Maran or Acolyte) shouldn't ever kill a neutral character (PC or NPC).
The important thing isn't that they are members of Fortress, but that they still act with a "good" alignment. That means that they shouldn't kill anyone for purely selfish reasons. Just mindlessly PK'g due to boredom, or killing a character just to steal their gear most likely wouldn't jive with most good-aligned characters, regardless of cabal. There are exceptions though, and there may be a viable RP-related reason for them killing your neutral character. And it's entirely possible that you have no clue what that reason is.
For instance.. as an Elf I killed someone once because they were wearing elf-skin gear. The guy I killed was a neutral ranger and had no idea why.. but it didn't mean my reason wasn't justified.