I tried one 'evil' playthrough . . . but honestly siding with the goblins still didn't feel like a compelling option. Halsin still seemed like the best option to solve our problem. Killing the goblin leaders (and a few others) was the most expedient method to resolving our problem. Perhaps I missed some dialogue with Minthara? She didn't seem to offer me any reason to side with her. Perhaps as an evil character you are supposed to be so excited about the growing power from your tadpole that you want to ally with the Absolute? Buy that isn't made clear by the story.
I still played as a selfish, pretty evil character, but killing a few goblins and freeing Halsin doesn't have to be considered 'good' by any measure. Mostly my evil character just wanted the tadpole removed and was looking for the most efficient way to go about it. I didn't care about either group.
Larian needs to introduce a compelling reason why you would side with Minthara and the goblins.