This idea was discussed and dismissed again and again many years before swiftstrike or even inherents were a twinkle in CF's eyes. The way I see it, is look at the backstory. Elves are supposed to be very goodly. So much so, that certain things will corrupt them unquestioningly.

Now think about the backstory of your character. Thief "I learned how to steal so that I could take the edge off of the dark one before he even knew he'd lost it." Warrior "I learned how to defend myself and if need be, make the first strike to protect the goodness around me." Assassin "I killed zounds of people so that I'd be good at killing. Oh, and I know how to kick as well."

One of these things does not belong, and I bet you can tell which. There are solid RP reasons that high elves do not become assassins. Half elves, those of less pure blood and descent, can.

The problem isn't that assassins can't be or do good, it's that they're far from the purity level of a high elf. On the note of elven purity, I've seen a handful converted to neutral and get reamed with the penalties associated for not _acting_ high elven. With the benefits of being a high elf, there is implied pureness responsibility. This conflicts with the concept of assassinations.

Note: The above is all my opinion, but answered this riddle for me years ago.