Jack Ruby, the assassin of Lee Harvey Oswald, has been consigned to a footnote of history. What Ruby did was to eliminate the only man who could explain the assassination of the US President, setting in train a mass conspiracy delusion that continues to cast doubt on the likely assassin to this day. Who was Jack Ruby, and what motivated him to do this murder that changed history? Ruby's fragmenting Jewish identity played a significant role in the events that led to the killing of Oswald and the loss of his sanity. In the year prior to the Kennedy visit, his mental state was changing. In a paranoid state, Ruby's behaviour was driven by a belief that there was an anti-Jewish conspiracy behind the killing of the President. In addition, his use of dietary amphetamines increased. Ruby's trial, followed by his time in prison until his death several years later, was characterised by increasingly irrational utterances that were filtered through his Jewish identity. A review of the evidence shows that organic factors affected his mental state. Ruby's killing of Oswald reflected aspects of the American Jewish migrant experience, his dysfunctional personality, abuse of amphetamines and the probably effect of a silent brain tumour.