I just don't see Astarion being evil, he comes across as aristocracy rather than a form of evil. He sees himself as above all others, similar to Lae'zel, and is interested in his own self gain. He wants to be rich, powerful and in control of his own life. Specifically, this is shown in his want to be free of Cazador's control and willingness to split his soul's ownership to accomplish it. Even his want to control the tadpole is born of a need to be in control rather than any grand evil designs. Sure the guy can be an ass and doesn't care about other people's lives but he's not psychotic by any stretch, none of the current companions are. His flamboyance (eccentric nature?) can perhaps come across as self absorbed and aloof but it never really reaches a level of evil, not in my mind at least. Lae'zel might, hard might, be the closest companion to being evil and that's only because she's a soldier with a fanatic loyalty to her queen, plus a hefty dose of pride. With the others; Gale is a moron, Wyll is good though has made at least one bad choice and Shadowheart is probably brainwashed. But, as Riandor says, none of the companions want to dance in blood or watch the grove burn because the druids didn't show enough respect, they don't find it fun to drive the Gnoll mad and make it eat itself after eating everyone else. While at the same time liking the Teiflings because they're polite and make nice music. Sure they can some of them can accept "evil" actions such as killing innocence or potential allies but for them its all kosher so long as you're making your way towards getting the tadpole out of your head. Truly none of them are psychotically evil or even just plain evil, which puts a crimp in a lot of the current evil choices and paths which are all kill anything that breaths. I mean if one of them had wondered aloud if the druids were all that tough when the lady at the steps threatens you that would've been a good poke that maybe you should just get rid of them. Little nudges here and there would work well, but for all the current companions that doesn't seem to be in their character. Granted such things can change, after all we've only got the first act right now, and maybe through the story Astarion or one of the others might "show their true colours". But as it currently is I have a hard time seeing any of the companions as evil in any real form.
The companions being more self absorbed jackasses rather than any form of evil can be seen simply in their whole reaction to the grove. Yes Wyll and Gale want to help, or at least Wyll obviously does. But Shadowheart, Astarion and Lae'zel are pretty much "This place have a cure? No? Well fuck this place lets get going". That said having something in your skull that could at any moment kill your and obliterate your soul does tend to redirect priorities but their tone sells the attitude.