My latest theory is that this whole thing was all about control.
JS said that she shot BS in the car because he told her to put the gun away and threatened to hit her if she didn't. She never says why she pulled it out in the first place.
IMO she pulled the gun out to make him shut up and do whatever she said. She was not going to kill him, just scare and control him (she was going to kill him later on, at home). But she was unable to control him and it pushed her over the edge.
She had an outburst in court because she wasn't able to control the testimony coming from the stand.
After she was caught, she suggested that she should just be able to pay a fine for killing the children. Even though she said it was the worst thing she'd ever done and even though she'd acknowledged how much it would upset PS, she still thought SHE should be the one to decide/control the consequences of her actions.
The scowl on her face, the way she stares down each witness is also an attempt to control them.
Exhibiting the TD symptoms during the videotape of her being transferred to the jail was an attempt to control our perception of events. Note how she looks RIGHT INTO THE CAMERA and then grimaces and opens her eyes real wide. I watched that tape about 4 times and I cannot see her face doing that at any other time.
Also she tried to control her psychiatrist. She continued to drink against his orders until she wore him down and was allowed 2 drinks a day.