Unfortunately, the scales of justice are not always fair.

In the United States, unfair interrogation techniques can lead to false confessions. These include sleep deprivation, withholding food and water, not allowing suspects to go to the restroom, interrogations that last longer than 24 hours, etc. Unfortunately, not all law enforcement officials are skilled at their work, and some are more interested in obtaining closure to the case to appease the frightened and angry community members than apprehending and punishing the guilty party.

Shoddy police work can also lead to consequential errors in the law enforcement field. It is not ethical to ignore promising leads and suspects because they do not fit into police's preconceived theories of the crime. Police in small towns are inexperienced with homicide investigation; they may contaminate crime scenes due to lack of knowledge and/or fail to collect crucial evidence from the scene immediately after the crime, such as DNA evidence, shoe and fingerprints, etc. 

It is a travesty that innocent people are ever executed for crimes that they did not commit, but it happens. It happens far too often in the United States of America, one of only five countries in the industrialized world that still administers that final punishment. DNA evidence has exonerated many people on death row. Alternatively, another person sometimes confesses to the crime long after the fact. We need to work feverishly in order to prevent these injustices from occurring. Sadly, the last words of some inmates that are executed by the state are protestations of innocence.

Law and Disorder: the Legendary FBI Profiler's Relentless Pursuit of Justice by John E. Douglas, 2013

I find John E. Douglas's books, mind and work fascinating. He pioneered the psychological profiling unit of the FBI, known as the Behavioral Research and Instruction Unit. Douglas recommends that all police interrogations be recorded in video and audio because detrimental techniques are often used. In the UK, police are not allowed to lie to suspects during interrogations. This has led to increased accuracy in convictions.

• John Douglas' web site
• Books about law
• How the UK Police Interviews Suspects