Cersei - has 3 kids. 2 good, 1 crazy. Abused and neglected by her husband who makes no secret of cheating on her, again and again.
Then her husband's long lost mate comes down, sticking his nose in and decides that whilst he can lie to the King about children's parentage, it's not the same for another Great House. He then plots to see if he can get the children killed.
Cersei doesn't take this and after years of abuse, knows the King has to be taken out of the picture. Ned, still clueless, decides to double down. He quickly decides once in a dungeon that it's better to retract his accusation. Cersei goes to lengths to prevent him from being killed.
Meanwhile Joffrey, who had his wrist ripped up by this man's daughter, then lost who he thinks is his father and then has the same man say his mother and father are incestous and his Uncle Jaime is his dad, decides to put a traitor to this death. Which is what Ned did in episode 1 to that man from the Night's Watch.
Everthing Cersei does is for her kids.
How can you leave out blowing up the Sept of Baelor?
That act of mass murder trumps any other act and nullifies any rationalization when it comes to her behavior. It was her Mad Queen moment, so yes, she is that bad.