I think we should first define what evil is. Ther are so many ways to go about this really... I mean for Astarion, exterminating the goblins is not morally bad, because goblins are vermin, and I agree with him. Killing goblins is not an evil thing to do. So we can establish that simply killing a bunch of deminhumans is not neccessarily evil. So then what is evil? I think evil is malevolence. When you not only do the deed but derive a significant amount of satisfaction from performing it to the point that it becomes an incentive for you to keep doing it. This leads to something of a psychopathic character, but I don't think most evil people are well adjusted at all anyway.
If you give a bunch of benefits to the evil path to compel people to walk it, you are suddenly turning evil into reasonable, in fact your character becomes more reasonable then evil.
Situation: I'm infected with a disease that can kill me at anytime The druids direct me to find an archdruid who may or may not be able to cure me The goblins tell me they will give me a surefire way to keep control over the disease is I do their bidding.
It's completely reasonable, and thus morally defensible to side with the goblins in such a case (this is not what happens in BG3, its just an example)
Evil is also something that stems from the inside. In the situation above, we don't yet know whether you are sorry to work with the goblins but are strung along due to your circumstances or whether you revel in killing innocents in their name. This must be exposed through dialogue and that's where companions should come in and comment on your deeds, so that you can respond in different ways to express your true intentions and motivations.
An evil character would always say something along the lines of: "I don't care if others got hurt, because I got my way" or "I enjoyed harming them, it caused me great joy".