Ahem...
.
You are talking about a character who as a child twisted her infant brother's genitals in his crib to torture him and probably killed the other girl who went to see the wood witch with her.
http://awoiaf.westeros.org/index.php/Cersei_Lannister#Early_life
In the first season of the show, Cersei sleeps with her cousin in order to have her husband murdered, betrays Ned Stark (the rightful regent of the kingdom until Robert's true heir becomes of age) to seize the throne for herself as Joffrey's regent, lies in order to have Ned's kids' wolves killed, etc.
If you are arguing that her saying to Jaime "HE SAW US!" is anything other than Cersei telling Jaime to kill Bran then I have utterly no idea what show you are watching.
Cersei has been a psychopath and sadist since birth. That much is obvious. She hasn't gotten more cruel over the course of the show, she has become more powerful and has had increasing opportunities to act out her cruelty.