Using the Paladin as a reference point, they lose all of their supernatural and spell-like abilities and spellcasting if they become non-good. However, their powers are divine based and not necessarily so much learned as "inspired." A monk that becomes non-lawful, on the other hand, doesn't lose anything. So, would an assassin lose spellcasting and any class abilities that are supernatural or spell-like at one extreme, or lose nothing at the other? Looking at the barbarian, a lawful barbarian loses their ability to rage. Using this as a basis, it would seem that the assassin should lose their death attack for alignment based reasons.
So, at this point, for the NPC in my campaign in question, I'm leaning toward not losing anything, but, the character is disinclined to use his death attack, or use poison, though he still has the capacity to.
Thoughts?