I've read the books only once when the third book came out, so I've forgotten almost everything, but the great lines to an extent.

Don't worry, Cersei in the show comes off as utterly dangerous and massively despicable. Very one-sided and vilainous for now (though IRRC, this will be balanced a bit later when we get to see what kind of father she has, something they've already prepared with Tyrion, and a few remarks by Cersei. She already comes off a bit as someone who was given no choice to be where she is, but also that she's as much a warrior in her way as her brother, but far more vicious)

The scene that nailed her character in the show (for a non-reading audience) is the one where she comes to the room to see if Bran will die, manipulating Catelyn's emotions (toward despair and pain, which was utterly mean) with her fake empathy derived from the death of her own first born (it came off as if she didn't care much about that, or worse that she killed that baby from Robert or knows her family did) - when the audience remembers all too well she gave the order to kill Bran, and was obviously very displeased when Tyrion told her he would live. Dangerous and cynical, and expert at hiding her real game is how she came off (it taints whatever she says afterward, you're not inclined to trust anything she says anymore). The way she traps Sansa in her webs is also quite obvious in the show - you even suspect a lot that after recent developments she must be the one who forced Joeffrey to pretend to be nice and finish to seduce Sansa away from her family.

What the scene where she gets slapped sparked isn't sympathy for Cersei (or a connotation of domestic violence), but anticipation that she coldly added the affront to the list of what she'll make Robert pay for.

For now I feel they've depicted her almost unilaterally as the show's real villain, and they milk it while it lasts. The time has not come yet to bring nuances to the character. IRRC there's no lack of events later to add depth to the character and show why she's like that.