Some people with a propensity for self-destructive behavior can’t seem to help themselves, President Trump apparently among them. Over the weekend and into Monday he indulged in another cycle of Twitter outbursts and pointless personal feuding that may damage his agenda and the powers of the Presidency.

Mr. Trump first expressed solidarity with the British people after the London terror attack, before managing to convert the mass murder into a referendum on his favorite subject, Donald J. Trump. He assailed London Mayor Sadiq...