"Our hearts go out to the families and friends of the people who lost their lives in the devastating attack in Christchurch."
He called it "a dereliction of duty."
The duke appeared to allude to the issue during a speech in Northern Ireland.
The Duke of Cambridge interviewed the historian and broadcaster at the World Economic Forum in Davos, Switzerland.
William also thanked the RAF troops for their sacrifices.
Kate and William met Vichai Srivaddhanaprabha's family during a poignant walkabout.