She even states that her heart is black, so I'd say that she's evil and is loving every moment of it.

I think it's more complicated than that. She's a goddess, not a mortal, and gods and goddesses of mythology tended to view morality differently from mortals. What we see as "good and evil" might not be how they see it, or how we apply morality doesn't apply to them.

That is, she the Goddess of Discord. It is both her nature and her job to love "glorious chaos," and to do everything in her power to cause as much of it as possible. And yes, this includes framing Sinbad and arranging the execution of Proteus so the kingdom will descend into power grabs and civil war. WE see it as "evil," but she's doing what the Goddess of the Chaos both loves and is supposed to do.

I agree she does "bad things" but not because she's malicious but more of its her nature and job and role in the universe

I agree (again) with Tygers and Lana. a lot of mythological gods are portrayed as neutral judges of mortals. they can be good but not necessarily nice. Eris was a thief, but she also called out Sinbad on stealing his own (very, very selfless) friend's fiancee, and kept up her end of the bargain. they are simply not morally like mortals because they AREN'T mortals.

and if her heart being black makes her evil, it makes Sinbad evil too. and he obviously was not presented as evil (even though I think he's rather despicable). Dark isn't necessarily evil just like good and lawful isn't necessarily nice (COUGH BATMAN).

Yeah, pretty much what everyone else said.
This is what's called "blue and orange" morality - basically, Eris operates outside of mortal standards for good and evil. Humans are her playthings, putting them in chaotic situations and shaking up their lives is just her way of having fun.