Why are people assuming Karlach is evil? You meet her while she is bleeding out after having been hunted by cultists, but if you convince her that you aren’t a threat she relaxes and there is nothing about her to suggest she is evil. She was forced into participating in the Blood War and enjoyed killing demons. These are not inherently evil traits.
Under the traditional understanding of D&D alignment, someone who is largely out for themselves is Evil with a capital E, even if they aren't into kicking puppies and children for fun. At best, Karlach may be neutral in alignment similar to Wyll and Gale. Which might be a step up from Lae'zel and Shadowheart, who basically hate every decision I make, but I'd like to have one good-tilting NPC in the party, you know?
