It’s a question of narrative function. She should be discussed as one—but that should not be where the discourse ends.
Cersei does awful things and the story doesn’t apologize for this. Examining her is not an act of apology: her narrative function is predicated on the reliable awfulness of her actions. She is a villain because she is vicious, selfish, often shortsighted, because she puts herself and the people she sees as part of herself before the rest of the world and hardly sees people beyond herself and her subselves (Jaime as her male arm, her children as her safety and her rebellion and the most valuable, fragile pieces of her life) as people at all.
But, see, that’s already deeper than “she’s awful” and I haven’t begun to unpack her as a person. She’s a villain and the text does not negate that by explaining her—it makes her a better character, a better villain, by forcing us to understand her, to look at every single one of her actions and see, even when they’re awful, even when they’re awful to her, that she could never have done anything else—not because she doesn’t have choices, but because she makes choices over and over that are consistent with who she is, how she sees her life, how she sees the world and her place in it. Some of those choices are bloody, brutal. Some of those choices are so tactically terrible they hurt to read (AFFC is the best book and the worst book). But they’re always hers, and they always make crystalline sense within the logic of her narrative and her perspective.
So, Cersei is, in order: chattel, because she’s a highborn woman, a valuable pawn; clever, if not brilliant per se, because she is Tywin Lannister’s daughter and a member of a house that values cleverness, that keeps its power through sly ruthless politics, that puts the knowledge of how to manipulate the world to its advantage before anything else; angry, because she knows power and has been told that she deserves it and believes she understands how to wield it but is nevertheless a woman and because Tywin Lannister is nothing if not honest about the exact political value of his children has been told that she is a gilded pawn from birth, the best gilded pawn in the kingdom but a pawn nonetheless; bolder than her brother, who gets to wear the armor and the heirdom but who wants nothing in the world except to spill blood and love her, and she loves him, and she loves him for loving her, but she is hungry for the world and he is given the world on a platter and that only makes her want more, seethe more, resent the infrastructure of the world more.
And that’s before we get into any of the actual circumstances of her life—before she’s married off to the hero of the realm who just happens to be a soldier with no claim to the throne and no lost love for her, before he calls the name of another woman in her ear on her wedding night (she just got the kingdom, just managed to get the most power a woman can have in the kingdom, and her husband the king lets her know even then that she isn’t important, that even when she’s settled into being a powerful woman she still somehow isn’t even the most valuable woman in the kingdom; it’s her night of ascension and she pales next to a dead girl), before the costs of queenship turn out to include violence and marital rape; before she hears the prophecy that she will rise as high as she can and then have everything she loves ripped away from her in shreds, so that even as she pursues the power she believes she was made for she forever hears the ghost of its future ruin in the back of her mind. Before her father is killed and Jaime deserts her and she’s the only person she believes can act as Tywin’s heir but is constantly told she isn’t doing enough (of course she overcompensates, of course she is too brutal, too rash, she’s not ruling, she’s proving). Before she’s given visceral reasons to want to revenge herself against the world and tangible needs to prove and overprove how much power she has and how well she can hold it.
Cersei is furious. Cersei has been headed for a fall since the moment she started to rise. And no matter how many people she hurts or how many ways she hurts herself, she could never function any other way. Cersei makes sense—narratively, structurally, every action is a proof of who she is, what she has been made and is made of. So of course it makes perfect sense to hate her for what she does. It makes no sense, though, to flatten who she is