There isn't much that I can write that others haven't touched on. A young woman kills a man, not enough is made of looking at who or what the victim was, but then again a life was taken, and if you don't want it to look like you killed him just to rob him, don't take his stuff. It's hard to fully paint a victim, but then again nobody does. Her defense lawyers never once shy away from holding their hands up and saying "Yes, she murdered him." Neither does she. But there are interesting aspects. Should a 16 year old really be tried as an adult? Is the sentence too harsh? Does the fact she is a victim of foetal alcohol poisoning really a defense?
On the last question, the documentary actually gives an interesting insight; nature vs nurture. Other reviews state she had a crap childhood. Actually, not really. She was adopted out of a bad childhood early by a loving, church going, caring mother who sent her to school like any other child. However, Cyntoia was not interested in education, choosing drink, drugs, partying, and sex with a much older boyfriend, running away briefly at 12 after being kicked out of school and scared what her mother would say. She went back to school but dropped out again, later confessing she thought she knew it all. These were all traits of her biological mother; drink, drugs, parties. A woman she appeared to have no contact or influence from prior to trial.
If you take nothing else from this, take the study of nature vs nurture as your foundation going into the documentary. It does make it more interesting.