It seems pretty clear that at some point in the past, Shadowheart was NOT evil. She probably suffered some terrible loss, and thus was easy pickings for recruitment by the church of Shar, the Lady of Loss. Then they wiped her memory and brainwashed her into their way of thinking, so now she's evil, but maybe she doesn't want to be.
At no point she ever stroke me as "evil", even in her current state.
Guarded against others? Sure. A bit sassy/condescending toward the protagonist at first? Also true. But that doesn't make someone evil.
Hell, I've seen videos of people playing the so called "evil path" of siding with goblins and wiping out the tieflings and even then she goes into a drunken depressive state, clearly uncomfortable with her actions even if unwilling to admit it.
