Letizia Battaglia's best photograph: mafia murder victims in Palermo

When the photographer returned to Sicily she didn't expect a war – but then the Corleonesi began a killing spree. Twenty years on from the murder of her friend the anti-mafia judge Giovanni Falcone, one of the great photographers of our time talks about her experiences

 'The prostitute in the chair broke the mafia's code of honour. Her body was now at everyone's mercy'