Oh and by the way I completely understand the evil requirement on assassin class - part of the requirement is killing someone expressly for the purpose of entering the organization. That pretty much means evil to me.
So why are assassins evil then and a paladin sect which requires you to destroy an undead/demon/evil dragon to enter is good (See D&D 3.5Es Vassal of Bahamut as example)?
I would not assign any alignment to assassins because it a world with objective alignment it always depends on who you kill. Would an assassin who only kills evil creatures be evil? And if yes how can adventurers, including paladins, who do pretty much nothing else than to kill "evil" creatures be non-evil?