Ryan Dancey wrote:
Evil societies will constantly be tearing themselves apart from within - such is the nature of evil. To advance their goals they'll be at each others throats.
Ryan, you seem to be forgetting that the concept of "evil tearing itself apart from within" is a cliche that only applies to the real world, not to the world of Golarion.
You are also making the common mistake of associating 'evil' with 'chaos'. Something that just isn't the case in this universe.
In a world of absolutes, it doesn't matter how evil you are, if you're lawful, you're not going to be backstabbing your allies.
No lawful evil character would ever go back on their word. If they did, they wouldn't be lawful. And with detect alignment spells abounding, it's childsplay for a guild of assassins to refuse chaotic members, and therefore maintain a degree of trust and loyalty to rival that of the paladins.
As an example proving my point, the Red Mantis Assasssins are required to be Lawful Evil in order to join. This faction, which Paizo placed in Golarion, contains absolutely no in-fighting despite being evil.