I set out to do an evil run for my first game. I made a lawful evil Githyanki slaver who was driven entirely by self interest and saw the races of the Prime Materium as tools to be used and discarded as needed.
I ended up doing what is pretty much a pure good playthrough because the evil route doesn't seem to offer you any reason to do it. This is entirely without meta knowledge, mind you. I played without knowing what any choice would lead to.
So let's go down my reasons for each decision I made.
1. I am a Githyanki with a tadpole in his brain. A cure is the priority. Nothing else matters. 2. I don't get involved in the druid politics. I learn Halsin might be able to help me. 3. I save the goblin so she can lead me to the goblin camp in search of Halsin. 4. I learn early on that this Absolute is tied to the tadpoles. Therefore it cannot be trusted. 5. I meet with Gut. I immediately do not trust her and do not ask her for help. She has a tadpole in her brain. She cannot be trusted. 6. Minthara orders me to destroy the grove. I kill her for having the gall to order me around and for having a tadpole in her head. 7. I find Halsin. He asks me to kill the goblin leaders and he'll help me with the tadpole. 8. I kill the goblin leaders and sneak out of the camp. End up getting the celebration with the tieflings. 9. Enter the Underdark. Side with Myconids because they offered to pay me.
So. Yah. There was not a single point where I felt even tempted to do anything especially evil. I got probably the most good guy ending you could get with my Githyanki slaver.
And this is because all the choices that seemed like they'd get me an immediate solution to my problem happened to also be the morally good choices to make.
Minthara needs to offer the player something for choosing her side. Right now she literally just orders you to do it as if you were an underling already sworn to her and the Absolute.
My suggestion? Have her hint at there being the possibility of learning to control the tadpole in your mind and harness its power for your own ends. That'd at least be something to tempt you to help her aside from boobies.