If you obtain a particular magic tome, you can sacrifice one of your allies for power, or sell one of them into slavery for even more power. Made particularly cruel for Grace, who actually used to be a slave, and Dak'kon, whose entire race were once slaves of the illithids.
You can release Coaxmetal from the Siege Tower with the Modron Cube, simultaneously unleashing a massive force of destruction on the planes, while also damning any Modrons who were still in the Cube's pocket dimension.
you can feed one of your allies to the Pillar of Skulls in exchange for information, condemning them to eternal damnation for the sin of believing you were their friend.
When you find Trias the second time you're in Curst, you can allow him to continue on his crusade to bring the Blood War to the Heavens, dragging countless innocent souls into a cosmic bloodbath for the sake of vengeance.
Fhjull Forked-Tongue has been magically sworn to give charity to anyone who asks for it. Because of this, you can effectively rob him blind of all his most treasured possessions, while he has to grin and bear it, and after you are done, you can murder him in cold blood for good measure while he isn't even able to properly defend himself,
You can continue your previous incarnation's abuse of Deionarra by manipulating her into giving you aid, even lying about loving her until you shatter her hopes of reunion once more.
just to name a few, the thing is its that kind of evil that's more like 'psycopathic' evil than your regular cartoon bad guy evil. you do legit evil stuff to people who legit care about you.
I would recomend doing a full evil run, and at the same time I would totally recomend avoiding it