SeranaEvil

Is Serana evil?

Mixed.
When her parents joined an evil cult and started worshiping an evil deity, she went along with them. Now, part of that is because she was their kid and kids don't really have a lot of options if their parents both start cult stuff, but she could have ran away and didn't. But on the other hand, telling someone to run away and start a life on their own in harsh skyrim is asking a lot. So it's hard to say just how evil it is joining the molag cult considering the circumstance, but I'd still say it was evil.

We never actually see her eat anyone, but she does allude to eating people. So we can somewhat reasonably assume she has eaten at least one person. Which is terribly evil. But if she hasn't eaten anyone in a while, that's some points. A vampire who stops eating people is trying to be good. She doesn't object to the treatment of the "cattle", which is evil, but on the other hand the circumstance is that she would just be ostracized or killed if she started making a stink about the cattle. So again, evil act but within a circumstance.

She saves the world though, so I think she gets major points on the "good vs evil" slider for that. Her mom wasn't noble. Her mom opposes Harkon because Harkon's plan might bring the rest of the races down on vampires and genocide them all. She likes the status quo of just eating some people in the darkness, but not enough to cause a concerted effort by the other races to stomp out vampire. So Valerica's motivation for opposing Harkon is pretty much entirely selfish. Serana, on the other hand, opposes Harkon so that "her family is known for something other than madness". She does it because she recognizes that her father is evil and must be stopped. She never really speaks strongly one way or the other about vampire kind or any larger goal, her father wants to kill her so she stops him. Her motivation is quite simple and quite noble.

So ultimately I think Serana is a very mixed individual. She certainly hasw done evil in the past, abides evil in the present which is bad, and is certainly not afraid of using the fact that she is a vampire to benefit herself. On the other hand, here she is joining the Dragonborn on a quest to save the world, she reveals a softer side that cares about her mom and about the Dragonborn, and we don't really see her doing evil.

Another important part of this answer is that she wants to get cured. if the Dragonborn brings up the topic of getting cured within the context of caring about her, she pretty easily decides to get cured (which implies she had been considering it), and when she does get cured, she sounds relived and her lines are about "being able to breathe again" and "the world feeling alive again" in a very relieved and almost appreciative tone of voice. She only refuses to get cured if the Dragonborn says they're uncomfortable around a vampire,or if they bring it up before she trusts them. I think this is important because it means she's willing to get rid of the evil taint inside her. Someone who says "naw fuck that, I like being a vampire" is probably evil, whereas the person who gets cured and is happy she got cured is possibly (but not necessarily) someone who is struggling with the decision to be a vampire and perhaps regrets some of it. Remember, she refers to being a vampire as a "taint" and a "darkness", and she refers to being a vampire in a positive light only in reference to it being a beneficial power. When talking about the experience as a whole, she makes a point to mention that it destroyed her family, and she wishes they could all be a family again.

So I think post-cure Serana, the one who is no longer affiliated with Molag Bal, the Serana who has finished her quest to save the world, the Serana who has learned the lesson of madness and evil and what obsession does from her father, is a good person. I think the vampire Serana who doesn't even concern herself with what is right or wrong and has not yet pondered whether to be a vampire is a bad person. I believe Serana is a person who changes at different stages of her life.