It's still like going Dark Side in KOTOR; just 'cause you can.
Which, let's be real, isn't a very good justification if you want the story to mean anything. I would've liked a setup more like Obsidian's Tyranny, where you could be considered evil regardless of what you do, depending on perspective, and doing good has a lot to do with minimizing the harm being done. What is the lesser evil between the stability, but utter ruthlessness and xenophobia of the Disfavored compared to the inclusive, but chaotic and amoral Scarlet Chorus? Does uniting the rebel factions actually lead to a better outcome or merely prolong the conflict with Kyros, leading to countless more dead, when bending the knee could save them? Is Kyros' Law so oppressive that they are worth fighting against, considering the scale of the destruction an Edict can inflict, not to mention the protections for magicians under the Empire rarely found elsewhere? These are pretty interesting questions, because different perspectives will lead to picking different sides and none of them (except the one where you turn on everyone) require you to have the Chaotic Stupid alignment. Currently, going evil in BG3 feels more Chaotic Stupid than anything. Even in-world it doesn't make much sense to me, since no one in their right mind would believe a goblin priestess could be of more help than a First Druid. At best you could say a realistic selfish or uncaring character would just not deal with the gobbo situation.
What we would need, I believe, is an option to be introduced to the evil side before we reach the Druid Grove, like running into a True Soul Drow (you could make them flirtateous as another incentive for good measure). Make it more of a "Do we follow this shady drow and possibly find a solution, or do we risk not learning enough in time because of our principles?" If we're going to have a "good" and "evil" playthrough, the evil one should at least be made to appeal to players and/or characters who are looking for the easy way out or the fastest way to power. And make us feel that picking the evil option is the only one available for a while, because tricking the player and/or player character to doubt that there will even be a "good" outcome is the best chance for a more realistic Dark Hat story if we insist on there being a "good" and "evil" side to this.
