Some relevant text to mull over...
[sblock=AD&D PHB]Assassins are a sub-class of the thieves, and they hove the functions of the latter as well as their own. Thus, to be an assassin, a character must have a minimum strength of 12, an intelligence of 11 or more, and a dexterity score of not less than 12. Assassins do not gain any experience bonuses for having high ability scores.
Just as do thieves, assassins have six-sided dice (d6) for determining the number of hit points (q.v.) they can sustain. Assassins are evil in alignment (perforce, as the killing of humans and other intelligent life forms for the purpose of profit is basically held to be the antithesis of weal). They can, of course, be neutral as regards lawful and chaotic evil. As mentioned above, assassins have thieving capabilities and their own ability functions. Because they can use any sort of shield and weapon, they are generally superior to thieves in combat.[/sblock]
[sblock=Complete Thieves Handbook]In any reasonably corrupt culture, there are those who wish to eliminate someone whose very existence stands in the way of their plans. To serve them there are Assassins: trained killers whose services are for hire. In the AD&D 2nd Edition Players’ Handbook, the idea of an assassin, a hired killer, has been divorced from any particular character class…
Most Assassins are of evil alignment. However, it is conceivable that one might be of a neutral (but not good) alignment. Player character Assassins, if they are permitted in the campaign, best fit this rare neutral description. A PC might be the agent of some monarch, paid to arrange the discreet demise of those who threaten the kingdom‘s safety. While this certainly is not good (in the moral sense), the character might regard it as a justifiable evil because of the deaths the action prevents by obstructing rebellion, invasion, or whatever.[/sblock]
[sblock=Dungeon Master's Guide v3.5]The assassin is the master of dealing quick, lethal blows. Assassins also excel at infiltration and disguise. Assassins often function as spies, informants, killers for hire, or agents of vengeance. Their training in anatomy, stealth, poison, and the dark arts allows them to carry out missions of death with shocking, terrifying precision.[/sblock]
[sblock=Heroes of Shadow]Assassins are cunning hunters and stalkers. They use a combination of lcthal poisons, exotic weapons, and shadow magic to ambush, confuse, and slay their enemies. Although many assassins are villains willing to kill anyone for the right price, some view themselves as Society’s caretakers. By culling Society of tyrants, murderers, and fiends, they ensure peace and stability for all. Executioners are assassins who use shadow magic as part of their arsenal. Their magic allows them to close with a foe and finish it off with their finely honed combat abilities.[/sblock]