Sixchan, you seem to have a concept of the D&D Assassin as almost like James Cameron's Terminator. But the assassin is not a construct - he/she is not a loaded weapon, incapable from distinguishing between good and evil. In fact, NO person would be like that, and still be a "person."
Well, I dunno about this. Aside from the flavour prerequisite, what are the chances that an assassin will ever be hired to kill someone completely innocent?
Actually, QUITE good odds. How about a paladin who dedicated his whole life to eradicating undead? How about a cleric who spent most of his life eradicating disease and feeding the hungry? These kinds of characters are very likely to have accumulated enemies that would hire an assassin to kill.
Now, how about someone a little grayer -- imagine a rogue who drifts from town to town, conning businesses out of large sums of money, and then moves on to the next town? Such a character might be gray, Neutral of some stripe, but
Worthy of assassination? I cannot see it. Yet, someone could well get ticked off enough to hire an assassin to do away with them, as an object lesson. What do they care? They don't have to SEE the assassination!
BUT -- the assassin does. The assassin has to see in graphic detail how they kill their targets - with the knowledge that these people have done mothing more wrong - than cheat a fat businessman. Or feed the poor. Or slay undead. To turn one's self off to these things, to be emotionless in the face of BEING ABLE to have emotion, is what fits the "Evil" qualifier. Burke once said, "all that is necessary for evil to triumph, is that good men do nothing." This applies intenally as well as externally.
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Now, totally ignoring the above, there is another reason why Assassins are evil. Monte/Skip/Johnathan designed an assassin class that fit the vision of an organization that is allied with evil powers in return for the knowledge of stealth, killing blows, and use of poisons. THe arcane lore, and the power to kill with a single striek was granted not by Special Forces, but by Dark Ones. Hence the requirement -- the DMG Assassin comes from an evil organization, not the position that the powers they get are necessarily evil - they are given by evil means, for evil goals, and are evil because of that. If a DM wanted a version NOT in the DMG mold, then it's fairly easy to change.