Note: You can be an assassin without having levels in the Assassin PrC (obviously).
Script Wrecked: I sort of think the class itself is pidgeon-holed, like you suggest, but being an "assassin" in the sense that you kill from the shadows for a payment of True, or goods, or "moral" satisfaction, or even bloodlust, is not beholden to such stringent requirements. I agree, though, the wording is funky when you compare it to the Lawful Evil alignment and consider that ostensibly there are (PrC) assassins out there that would be lawful evil.