"Let him have it Chris" was a phrase Dave was often shouted simultaneously slapping his hands in outrage. This sentence had cost Derek Bentley his life.  Derek was the last man in England to be hanged in controversial circumstances on 28 January 1953. 
Police Constable Sidney Miles, was murdered in the course of a burglary attempt. “
 At the time, Christopher Craig, then aged 16, a friend and accomplice of Derek, was accused of the murder.  Derek Bentley aged 19, was convicted as a party to murder, by the English law  principle of common purpose, or “jointenterprise”. The jury at the trial found him guilty based on the prosecution's interpretation of the ambiguous phrase "Lethim have it" ( Derek”s alleged exhortation to Chris), after the judge, described Derek Bentley as "mentally aiding the murder” (More detailed account of the burglary at the end of this entry) Christopher Craig could not be charged as he was still a minor.
46 years later it was acknowledged that Derek was the victim of a terrible miscarriage of justice.  In July 31, 1998  when Britain's high court overturned the conviction against Mr. Bentley,  it was finally aknowledged that the trial judge had presented a one-sided account of the case to the jury:
''It must be a matter of profound and continuing regret that this mistrial occurred and that the defects we have found were not recognized at the time,'' the court said. Derek was given a posthumous pardon in 1993. Dave used to shout “Let him have it Chris” and talking about this miscarriage of justice to anyone who would stop to listen, from the 1970’s onwards.  He definitely felt  Derek Bentley had intended the words to mean the literal meaning of "give him the gun, Chris" instead of the informal meaning of "shoot him, Chris".

Although Dave often seemed to revel in outrage and tales of murder or revenge he would often try and reconcile things said with a more humanitarian take on what motivated people. If you read about this story in Wikipedia you have to click through quite a few  links to find that this misscarrage of justice was carried out on a man who had a series of health and developmental problems. In December 1948, Bentley had an estimated "reading age" of 4 years, 6 months. David’s reading age was 4 years. After Derek’s arrest in November 1952, IQ tests were administered to him at Brixton prions . He was described there as "borderline feeble-minded", with a verbal score of 71, a performance IQ of 87 and a full scale IQ of 77. 
Peter Thomas always added that Derek had special needs and perhaps that is why this particular injustice resonated with them both and Dave talked about it so often.  Dave was also very outspoken about being completely against the Death penalty as a form of Justice.
This is Detailed account of the burglary:
At around 9.15pm, neighbours called police after spotting Craig and Bentley climbing over the gate and up a drainpipe to the roof of the warehouse.
When police arrived, Craig and Bentley hid behind the lift-housing. Craig taunted the police. One of the officers, Detective Sergeant Frederick Fairfax, climbed the drainpipe to the roof and grabbed hold of Bentley, but Bentley broke free. What happened then is uncertain: police witnesses later claimed that Fairfax ordered Craig to "Hand over the gun, lad" and Bentley shouted, "Let him have it, Chris". Craig fired, striking Fairfax in the shoulder. Fairfax was nonetheless again able to restrain Bentley, who told Fairfax that Craig had further ammunition for the gun. Bentley had not used either of the weapons in his pockets.
A group of uniformed police officers arrived and were sent onto the roof. The first to reach the roof was  Police Constable Sidney Miles, who was immediately killed by a shot to the head.