HBO - Game of Thrones SPOILERS INSIDE. you have been warned

[Chain]

Now a character may be able to atone for evil in their past through future actions and I think Jaime may be on this road and he is being made to pay for past crimes. Cersie however is an evil, calculating bitch with no remorse for any of the pain and suffering she has inflicted on those around her in her attempt to seize and hold power.

What makes you think Jamie is on his way to redemption? If anything he has less remorse for his actions than Cersie - and he was the one who actually threw Bran from the window! In the books at least (I forget whether they portrayed it in the show), Cersie is upset with him for that and insists they could have kept him quiet without killing him. What's more, Jamie can't even claim the excuse of trying to protect his children because, as revealed from his point of view in book 3, he doesn't give a fuck about them.

In fact, Jamie is so remorseless that he seriously considers abandoning his white cloak, marrying Cersie and then marrying Joffery to Myrcella the way the Targarians had done for centuries. Cersie refuses him and even consents to be re-married to someone else although she doesn't want to because it's what's best for her children.

As for the assassin
[spoiler]He dies before it can be confirmed, but it's hinted strongly at several points that Joffery sent the assassin out of a mixture of cruel stupidity and a desire to impress his "father" Robert who had said it would be kinder to kill Bran than let hm live as a cripple.[/spoiler]

Should Cersie be held responsible for her actions? yes of course she should, but she is demonized beyond the scope of her actions (which, when you cut right to it, amount exclusively to falling in love with her brother and arranging the death of her physically abusive philandering husband) because she happens to develop enmities with a few of the most popular characters and has the misfortune of being a woman.