When Thomas Mair was arrested for the murder of Jo Cox MP in 2016, he told police he was a “political activist”. Asked to identify himself in court, he said: “My name is ‘death to traitors, freedom for Britain’.” When Ali Harbi Ali, the man who murdered David Amess MP in 2021, was interviewed by police, he claimed to have been serving a “just cause”. Asked if he thought his actions were rational, he replied: “If I thought I’d done something wrong, I wouldn’t have done it.” Neither man was a member of a banned organisation. They took solitary paths...