Two years of investigations climaxed last week when the MP confronted News International chairman Mr Murdoch at a hearing of the Culture Select Committee - and called him the first Mafia boss not to realise he was running a criminal enterprise.
His jibe produced sharp intakes of breath from others in the room.
"I expected criticism but I think most people know I feel pretty strongly about this," he said. "It's something I've spent two years on and maybe I was revealing a frustration with the process."
He was nervous at the committee session. "I also had a huge dilemma as I had information from Neville Thurlbeck [an ex-News of the World reporter] that he'd asked me not to make public," he said. "In a sense I felt obligated not to reveal it but there was a higher obligation to the public interest to get it out."
A week later he is still unsure whether Mr Murdoch told the truth. "He has admitted wrongdoing happened on his watch and pointed the finger at others for failing to inform him," he said. "But why on earth didn't he ask some basic questions?"
Mr Watson, 44, is a complicated hero, with the ebullience of a schoolboy and, ironically, the frame of a Mafia heavy.