Imo, the problem with Minthara is part of a bigger problem with evil paths in this game. If you try to play a neutral evil character, something of a pragmatic villain who does whatever benefits them the most, you'll end up picking almost all of the good choices. Like, what do you get for killing Nightsong? A cooler spear? If Shar at least gave you and your companions permanent resistance to the shadows, or maybe ordered her shadows to help you assault Towers, then killing Nightsong would make some sense.