To be honest from what i've seen and read of others doing evil playthroughs half the evil outcomes don't even work or fizzle out to nothing.. You kill the druids and the tieflings then what? become master mindflayer or boss goblin leader.. nope.. nothing most totally evil endings seems to bug the game out..
Are you aware that you are evaluating the "end result" after the first third of the story?
I'd argue that the "end result" of the first third is much more relevant to evil playthroughs than the full story ending. Typically, "evil" should have immediate benefits but long-term detriments. (e.g., You kill someone for their stuff. Great, you got their stuff! But now the town is after you and you either have to bribe guards, leave town, or get arrested. Or: evil overlord amasses power takes over the country. But eventually, inevitably, heroes come to kill the evil overlord)
In BG3 Act 1: The evil route should result in you having gotten significant power. Evil is tempting for this reason! Characters don't just decide "hey, I'm going to be evil now." Characters make evil choices because they are selfish and the evil choices benefit them at the expense of others!
But, the current ending of the evil act is "You killed a bunch of tieflings and druids, basically acting as our servant/murderhobo, and then you get betrayed" (I didn't play this "evil route" so I might be mistaken)
tl;dr. Any evil route should have immediate or at least easily apparent benefits. The "evil route" in BG3 act 1 has basically no benefits that your PC would know of.