The whole case seems quite odd. There seemed to be quite a bit of sympathy for her and that her being abused as a child wasn't being taken into account but from the (little) I've read it was a pretty abhorrent murder she committed and the abuse she suffered doesn't seem directly relevant (i.e. she didn't murder a perpetrator),
It's relevant because she had little real parenting. Her parents were alcoholic. She was raped as a toddler, at age 11 and 17. She tried to kill herself when she was 12 and was sent to a Juvenile Evaluation Centre at 15 following breaking probation terms (she had been caught stealing candy). She spent a year there due to bad behaviour and prior to her admission had had to be removed from her home after her mother's boyfriend sexually abused her.
JEC did turn her around and encourage her to train as a teaching assistant before enrolling in the Job Corps programme. Whilst there she became convinced that Colleen Slemmer was trying to steal her boyfriend. She and her boyfriend murdered her in the most horrific way possible.
The US Prison system made no attempt to rehabilitate, and the "sympathy" is not for the murder she did, but for the fact that her mental state and history of abuse (plus PTSD and bipolar) was not taken into account when she was tried. Arguably it would have been more appropriate for her to have a psychiatric evaluation and treatment rather than a life imprisonment. The argument is not that she should have been released but that execution was not appropriate. Her boyfriend who was just as culpable was not sentenced to death because he was a couple of months younger.