Thornir Alekeg said:
I'm surprised nobody has mentioned Robin Hobb's books. The main character is an assasin. He is trained in stealth, observation, spying, poison, killing silently with small concealed weapons etc. etc. But I would not for a second say that he is evil. I would likely categorize him as good. He kills and spies for his King. The people he kills at times are not evil themselves, but are political problems whose death would greatly simplify things for the Realm, and sometimes he does not kill at all, but uses intimidation, diplomacy and other tactics to put an end to the problem. I see no reason an Assassin PrC could not be easily adapted to fit that mold if it suits the character.
Dito.
(as soon as I saw that thread title, I was thinking to myself - latest that someone's gonna mention that series is the 18th post - and I was right

)
As for a constructive addition ... in Steven Erikson's 'Malazan Book of the Fallen' there are several different and fairly unique groups of assassins.
There's one guild in a big city, which is purely for hire, but actually serves as the valve, by which pressure can be relieved, so the political backstabbing (pun intended) doesn't turn into a all-out war, but is resolved cleanly and by professionals. They are lead by a great sorceress, who equips them with lots of magic items. In DnD terms they'd probably lawful evil, killing without regard for the individual.
Then there're several small, hand-picked groups of assassins serving the 'holy protectors', the rulers of a desert country one continent away. They are probably the most like the historical hashashin with the religious background. Alignment would be chaotic neutral fanatic by my estimation.
Finally there are the assassins of the Malazan Empire, the Claw. Trained intensively as much as mages as to be assassins, their abilities are rightly feared - in fact a rumour of an intervention can be enough to make rulers flee the city/whatever. The greatest among them take the fight to the gods opposing the Empire, for their loyalty is only to the empress. There also exist remnants of their predecessors, the Talon, who were serving the last emperor and even without any magic were defeated only by sheer numbers. In DnD terms both would be lawful neutral - serving a single cause with utter loyalty.
Well, there's one more group ... the Tiste Andii assassins are of an immortal, otherworldly race and as powerful as this implies. Fortunately they are too apathic, too bored by their countless millennia of existance, that they only ever get involved, when their ruler tries to alleviate this boredom by joining just another conflict between mortals just for the heck of it. These people are neutral bored.
As was stated by others before: being evil is just one facet of being an assassin. There are so many motivations and almost all can be served by sudden and convenient death of others ...