I have loads of criticisms re: the rule implementations but I'm happy with the story so far.
Two points -- 1) Faerun just isn't an evil friendly setting 2) the evil playthrough doesn't have the narrative deficits critics claim. (even if it has bugs)
1)
As @kanisatha has said this is keeping with the FR setting which has always been about heroism. Greyhawk, by contrast, was about grey morality with "neutral" being the default. The evil path should be there but I don't think it should be as compelling as the good path. It should be like BG1&2. We see how few people actually enjoy the path in Larian's numbers, the sales numbers of Tyranny, the failures of Beamdog and it's evil edition etc. Fully supported evil playthrough is a very vocal, very niche market.
I want evil to be in the game -- otherwise my good actions are meaningless -- but I don't want half the game to be dedicated to a path I and most people will never follow.
2) The evil path in BG3 is the same path that Sarevok walked down. He thought he could use the harness the power and not have it harness him. (And he was wrong)
Astarian is key to understanding the evil path. The parasite has freed him from his master, allowed him to walk in the sun, to enter houses uninvited and he wants to know if its possible to take the benefits and leave the rest behind. What if the plan isn't to make you mindflayer? If the absolute wants to him to become the strongest vampire lord in Faerun, well, great! So what if the masters are mind flayers? These masters seems less demanding than the last.
And there are reasons to believe that the tadpole can be harnessed. The absolute wants the survivors of the crash dead. Could it be that the process was interrupted and the protagonists have more control over the "absolute powers" than the absolute intended? Perhaps there is an opportunity here -- infiltrate the new church, find the weaknesses and exploit them.
And you can just be seduced and tempted. I mean really early on you get a powerful set of gloves soon to be followed by a shield and amulet that can only be used if you take the brand and shout the absolute's name. Add to that powers from your seductive dream partner and the promise of drow bewbs and you've been seduced by the power of the dark side.
