Yes you can romance Shadowheart on the evil path.

[wowbloodelf]

I killed the grove, was still nice to shadowheart as much as i could be. by the time i got ready to go to the mountain pass she came and got me at camp and we drank wine together and kissed on the cliffside. When i killed Astarion at camp, and talked to shadowheart she claimed she might start stabbing people through the heart preemptively. damn shadow and i thought i was evil! That made my evil heart love her even more.

[ACS36]

Your companions don't care about your actions at all when it comes to romance options.

THEY can completely dislike you, and you can still romance them if you choose the correct dialogue when talking to them.

People are confused about this aspect of the game. Approval/Disapproval doesn't matter at all for romance.

[wowbloodelf]

not entirely true, if you kill the grove i know two companions that will leave your group or not join your group later. I dont know if that will happen anywhere else in the game. You wont be able to romance them after that.