Think of it this way -- the Spirit Knight has sworn to protect the area against all intruders and so he will attack any who venture in. The Whitecloaks were the same way with the idea being (I believe) that anyone who wasn't with them wasn't really sworn to protecting the Light and so was an enemy.

Heck, at that level, it's not even much different than CG in Outlander occasionally battling a OG in Tribunal or a good-aligned villager going after a good-aligned mage.

That's not the same as the general, feel-good, type goodie who wants to help everyone who isn't evil and only wants to kill truly evil.

And frankly, at your level the Spirit Knight is only annoying because of the hold spell and because you happened to be in a pk. It's not much different than fighting next to a death trap and getting the other player to flee into it.