Shadow heart doesn't seem evil at all, considering she likes it when you save the Tieflings, and the beginning of her romance was kind of lovey dovey.
Yeah she's clearly not, at best she rping being evil, but she pretty much universally likes clever solutions to problems and limiting bloodshed/violence.
Also love any option to be nice to animals
Clever solution to problem and avoiding fighting is a very Sharran things to do. They are into subversion and manipulation, not direct confrontation.
People really need to stop thinking that "evil = resolve everything with violence, can't like pets and can't love/like people". That's not how D&D alignment describe evil.
Shadowheart is Neutral Evil, a pragmatic who doesn't have a problem with anything as long as it advance her goals and makes her gain power. If you don't believe me, use your tadpole and in the dream #2 post-conversation, pick "I wanted it all" and then the "which part you liked best" option.
She wants power.
