I myself see Faerun as a neutral plane put in a sandwich between good and evil planes.
Now that I think about it, are there any evil centered campains in this setting?
My 2e sourcebooks are in a box somewhere so I can't get quotes or page numbers but it was said, in pretty explicit terms, that Faerun was a land of heroes. The studio that made BG repeatedly said they had no intention of supporting evil playthroughs.
Like all editions since 2e had it controversies, Faerun had replaced the morally grey, Neutrally-aligned Greyhawk setting as the default setting .
Faerun wasn't well received at first because it's new status coincided with the decision to remove Demons and Devils from the game. Also, TSR's decided to reach out to and hire some fundamentalist Christians and this upset fans who thought TSR should be pushing back against the satanic panic, not making concessions to the fundis (both the Dragonlance and Nethril settings were authored by fundamentalists). Fans protested and the demons and devils were brought back with different names *
Greenwood pumped out supplements at a pretty quick rate and most fans warmed to Faerun.
The people who preferred morally grey adventures eventually got the upper hand when TSR/WotC announced 3e. Greyhawk was, once again, the default setting and WotC started making supplements to support evil parties like the book of vile darkness. Monte Cook became an important voice in the new WotC and his blog became a place for people who disliked heroic narratives to gather. At the time you played Greyhawk if you liked to play evil or neutral, Faerun if you liked to play good.
4e happened.
5e happened, advocates of heroic adventures like Greenwood and Salvatore played an important role in telling the resurrection of Faerun story and there we are.
Is it entirely clear that Faerun is still a setting where heroes vanquish villains and monsters? No. There's no crystal clear paragraph that says "Faerun is a heroic setting" It's just the history of the setting and the fact that the members of 'team evil' have either left WotC or stopped making public statements about supporting evil parties.
On your question - 3e Greyhawk was neutral with evil playthroughs fully supported, The Dark Sun campaign was heroic but 'evil' magic is stronger than 'good' magic. It was setting a reminiscent of the Mad Max movies - one type of magic is good for the environment and is pretty difficult to use, the other destroys the environment and is pretty easy to use.
*(during the Satanic Panic fundis believed that DnD taught kids the 'real' names of demons and how to summon them)
