Any Forgotten Realms discussion of evil that doesn't include the role of the gods is flawed. It's just a mistake to import contemporary notions of good and evil into this fantasy realm. In our world we don't think of ontological evil. I, and most people, don't think that people who do morally reprehensible things are being influenced by cosmic forces and that these evil acts take on an ontological form in another dimension of reality. But that's not true of Faerun. In the realms, evil is real. Every murder produces a divine tear in the throne of Bhaal and Bhaal collects and treasures every single tear.
A priestess of Bhaal who murders everyone in the grove is not playing "stupid evil" she is correctly playing to type. In this fantasy world evil for the sake of evil is the default motivation of villains because, unlike our world, evil is an ontological reality.
Anyone watching American Gods? It covers this pretty well. Every sacrifice carried out in Odin's name makes Odin that much stronger. In Faerun, every priest of Bhaal exchanges their god given gifts for acts of worship designed to strengthen their patron deity -- acts of worship such as the murder of innocents.