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.
Yep, I was playing a githyanki wizard, so there was no incentive whatsoever. The reasonable choice seems to be to shrug and move on, leaving the goblins and the tieflings to fight it out, because an evil character would not care. And possibly assassinate the absolute leaders on your way out, as retaliation for daring to try to kill you.