christian longo has been writing letters to people from his death row cell .
believes ` some actions are so terrible that nothing can ever atone them '
in 2001 he killed his family , stuffed them into suitcases and dumped them .
three children and his wife mary jane were all found by police divers .
fbi tracked him to cancun where he was partying with a german woman .
was posing as shamed new york times reporter michael finkel .
he was brought stateside and sentenced to death at the end of month trial .
movie starring jonah hill and james franco about his relationship with the journalist has been released .