If children are not challenged during their formative years (and are kept in isolation) they will experience emotional, physical, and mental challenges that they may never overcome. Talking and interacting are critical to early development. To deprive someone of human contact (especially when they are young) is a terrible thing to do to a person.
Although the parents are responsible for what happened to their daughter, I find that there are plenty of others who could shoulder some of the blame for her well being after she was discovered. While her case was an interesting one for science, I find that the relationship between the scientists and Genie to be misguided at best and parasitic and exploitation of a minor at its worst. Genie is a human being, and she needed decent care... she was used as a lab rat and discarded by the scientific community when the research was acquired.
I also find it deplorable that she was placed in a loving home after she was found... instead she was abused and mistreated in foster home after foster home. Perhaps society failed her as well, and placed her where the light would not shine on her again.
We find someone and save them only to put them back in a similar hell that they experienced... I cannot describe how badly I feel Genie's case was handled.
Even though their actions were very evil, I don't see how retributive justice will somehow make amends to the harsh treatment that she endured. Her father is already dead, and I don't think his body or memory should be dealt any more penalty. I hope that her mother is brought to proper justice (I assume that she also had a role in the neglect), but I don't think she should be executed. Instead, she should serve a sentance that is fair and just (I personally don't know what that sentence would be).
I cannot say that I am surprised at the neglect that she suffered. If you truly wish to see human suffering, look at the poorest regions of the world that are neglected.