Turns out you don’t need to permanently kill Karlach for the best robe in Baldur’s Gate 3
There is actually a way to get the Infernal Robes from Mizora without permanently killing Karlach in Baldur’s Gate 3, but you will still have to get rid of her—temporarily.
In BG3, several items can be classified as the best gloves, weapons, and armor. For robes, one of the best ones you can get is the very rare Infernal Robe. This robe grants you resistance to Fire Damage, plus one to your Armor Class, and you also have a level four fire shield that not only reduces all incoming Cold damage by half but it will also deal two to 16 Fire damage to all enemies that hit you with a melee attack.