The recent murder case of a Welsh man who murdered his wife while dreaming, is certainly getting a lot of media attention. Could this really happen? Was he actually just malingering? One of the primary phenomena that occurs during REM is the neurological inhibition of voluntary nerve impulses during REM sleep. This renders the dreamer immobile and therefore, unable to move. So how could one possibly commit murder if this is the case?
With many sleep disorders (parasomnias) they are associated with non-REM dreaming states and therefore, acting out without the dreamer knowing it, is possible. In the case of the Welsh man he stated that he acted out a violent dream and unknowingly killed his wife. This has come to be known as REM Behaviour Disorder. These disorders occur, unlike in normal conditions, where behaviours occur that are not the norm. People have been known to display bizzare behaviour while sleeping (e.g. walking, talking) which can even include murder.
Though this is a very interesting case, especially for sleep and dream scientists, it should be noted that it is extremely rare and it is a "disorder". That being said, few people will ever act out in a violent manner while dreaming. In the vast majority of cases, the REM response will be completely normal and the dreaming function will do exactly what it is meant to do for the dreamer.