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.
Damn that sucks. No wonder most DnD properties seem to be overwhelmingly reductive about being evil/bad/not good. All the time it's "Evil is when selfish or murder or mustache-twirling villain."
Evil is not easy to play or roleplay. It's advanced mode when it comes to D&D. People who are bad at it often descend into cartoonish evil or edgelords without purpose. Murdohobo isn't evil, its just a big "fuck you" to the idea of roleplaying anything. It's a child toppling over another child's blocks to see what happens. Fun, in a crass sort of way, but requiring no skill or knowledge.
Regardless of what the setting is - we have a right to roleplay evil characters if we have the intelligence and skill to do it right. Broadcasting alignment is one of the ways that sort of thing gets undermined.
For the record, an evil player should understand they need to work within their party's social dynamic. If they become anti-social to the point of needing to create drama or attention then they are not doing a good job and should be invited to leave.
Larian is doing it right, the 5E system is doing it right by downplaying alignment for PC's. I celebrate the end of that Stupid Evil phase of 2e.
