Ok so now it's clear that the "evil" path means...
...using the tadpole to gain more power. Allying yourself with this evil faction you know nothing about that is trying to kill you.
It makes no sense.
Motivation is the most important thing in writing evil characters. If I play evil it's an intelligent, methodical evil that has a goal. That goal can be personal power, but I wouldn't ally myself with whatever evil faction comes along or keep such a meaningless power source knowing it can kill you. Furthermore, the goblins and the other weak-willed misguided individuals don't exactly scream "join us". I'd rather ally with the Zhentarim who are an established faction with smart people.
Having a shadow magic infused tadpole in your head and being a part of someone else's unknown plan means you are not in control. The first thing a sensible evil character would do is get back in control. Besides, a slimy parasite in your brain that is largely unknown is simply gross. And you know it wants to kill you. Say the creator of the shadow magic decides to kill you. They can just dispel it and you die a horrible death and turn into a Mind Flayer. Getting the tadpole out is the only sensible thing anyone would do. And also the perfect motivation to do evil things. But this is not even an option in BG3. I would totally wipe out the Tieflings if the Shadow Druids would remove the tadpole in return. Getting cool powers seems more like it's for gameplay reasons to get more Bonus Actions and doesn't outweigh the fact that you can be killed or controlled by the shadow magic tadpole.
So my evil Drow Warlock ended up killing the other evil characters for Halsin because he seemed like the best or only option to remove the tadpole. I was really annoyed this didn't happen. And the parasite is gross, I just want it out regardless of whatever.
That was my conclusion and wrote a similar post. Evil does not mean you suffer serious mental retardation. Evil is simply the methodology applied to forward your motivation.
Goal - cure a plague
Evil - Requires slaves, power and material goods as reward, not interested in colateral damage incurred or methods used to obtain cure. Torture, theft, murder....preferable. No reward? Bye bye. Ends justify MY means.
Neutral - Similar rewards unless this demand complicates scenario. Will consider all options and methods if said methods increase chances of success, can be subjectively good or evil. The ends justify the means.
Good - Altruistic nature may refuse reward, may refuse task if plague is "seen" as divine or just. Will apply morality and feelypoos even if it means everyone dies. The ends do NOT justify the means.
Good in this context is a limiting factor. Morality makes easy solutions ridiculously convoluded in most cases. Neutral people understand that sacrifice is sometimes necessary to obtain a desired result. Evil people don't give a shit as long as the rewards outweigh the risks, they may even obtain the cure to blackmail the client...then kill them.
Good, neutral and evil characters with the same goal may come to the exact same conclusion. In this case "the tadpole" is clearly a controlling factor to all allignments and everyone would want it gone for different reasons. By joining forces with a group you know for a fact is controlled by tadpoles you destroy a potential cure for no discernible benefit, it is utterly idiotic.
[spoiler] The vampire wants to control the tadpole because it offers him a solution to a his slavery, understandable motivation. Nobody else does.