Horizon - How to Commit the Perfect Murder (2007)
"Modern forensic science should make it impossible to commit murder and get away with it. But how easy would it be to outfox the detectives? With the help of top forensic scientists, and real-life murder investigations, we explore whether it's possible to commit a perfect murder."
Forensic science is just a methodical study of the body and crimescene... it has been developed for over 100 years now, and it is not that it is not impossible to commit a perfect murder, the human error factor, and unpredictable behavior of the person being murdered, stress factor for the murderer, and vitnesses make it hard. One makes mistakes easily... besides I don't think that people committing murders are in a rational state. Governments (almost all the secret services of the world) have done lots of murders, but they have a lot better resources, and training.