David Swindle has spent his working life catching killers.  Taking dangerous people off our streets and protecting the public.  While relatively uncommon, killers coming in pairs is not totally unheard of. It even has its own term "folie a deux" or the madness of two. Many people will know the names Fred and Rosemary West, Bonnie and Clyde, Ian Brady and Myra Hindley but few will have heard of Rachel and Nyomi Lee or Sarah Williams and Katrina Walsh?

As David says, partners in crime can be particularly dangerous.  Sometimes the drive to commit crime is equal in both but often, one can influence or lead the other.  Frequently there is another tie such a love or ambition and that merely serves to strengthen the bond.