A lot of the 'roleplaying info' text assumes that you will play your character as a 'noble and good rebel'----opposed to the racial type? For instance---all the examples of playing a Drow (from the FR Player's Handbook) indicate that the PC opposes the Drow races evil tendencies.
But what if you want to play a bad ass evil Drow who goes back to Menzobarrenzan every once in awhile and reports back to your Matron Mother?
Note that the rules can change, and do, from table to table.
The classic example in 3.5 is, of course, the Eberron Campaign Setting, which enables and encourages the play of races traditionally deemed monstrous, e.g. orcs, goblins, hobgoblins, gnolls, minotaurs, and others.
The western orcs, in particular, are deemed a highly spiritual race; whether serving as Gatekeepers--the druid order responsible for defending the world against extraplanar threats--or paladins of Kalok Shash, "the Binding Flame," responsible for ensuring that the hosts of the Demon Wastes remain there, most orcs described in the setting are either genuinely heroic, or contemplative mystics.
Similarly, it's not unheard of to find mercenary bands of gnolls available for hire in any of the larger cities in the setting, and while not fully integrated, most cities have a large population of goblins that undertake all manner of occupations from shoeshine boy to investment banker, though the banking industry
per se is still dominated by dwarves.
So--if playing monstrous races works with your setting, do that. Playing a dark elf working with surface dwellers
in order to spy upon and scout out the surface world is not at all out of character. Likewise, a shadar-kai in the world is likely simply seeking new experiences to deal with its racial sense of ennui, whether or not in service to the Raven Queen.
Playing a goblin working with other races requires even less justification. Similarly for gnomes, minotaurs, or any other race not assumed to have its own somewhat insular civilization.
It's all simply a matter of how your campaign setting works: in a really simple hack-and-slash environment, where civilization exists only as a place to exchange coins for magic items or vice-versa, the race of a player character only matters in a purely game-mechanical sense, and none of this discussion is needed. If your setting assumes that all of civilization was more-or-less wiped away by a wave of marauding gnolls (c.f. the fate of the Empire of Nerath, in the Points of Light setting), then it's going to be harder to play a gnoll character than, say, a gnome.
In a larger sense, the assumption in the rules that the player characters are good is generally true of reader-identification literature in general: it's not unique to Wizards of the Coast, or even role-playing games. Simply looking at literature generally, the character with whom the reader or audience is expected to identify is almost always what we would characterize as of good alignment.
Even in situations where the reader-identification character is explicitly a sociopathic serial killer--e.g. the television series
Dexter--the character is also usually
good, in the alignment sense of Dungeons & Dragons: despite being perfectly willing to butcher living victims, for example, Dexter adheres to a code that calls upon him to never harm an innocent.
That the rules even
permit--not encourage, but merely permit--the play of evil characters is something that continues to render Dungeons & Dragons quite suspect to many people outside the hobby.
—Siran Dunmorgan
)---and almost none of the base races/classes had any 'Dudley Do-right' language.
