Karlach and Wyll having evil endings sound so out of character, but the power of the player/dungeon master can do anything. This even includes turning Astarion and Lae'zel into saints.
Bro Choice 4 is especially dark. Like it's so uncanny and disturbing even among the already unnerving evil endings. At the start, I was thinking "Oh at least the flaming fist and the kid got away" but no, it's just stretched out pain.