"But when that is followed by a Trump tweet criticizing the mayor of London for a reason Trump misunderstood ..." -- Stephen Cohn
President Trump didn't misunderstand anything. Trump has a personal grudge against the Mayor of London, and acted accordingly. The mayor of London was clearly talking about an increased police presence in the city when he asked Londoners not to be alarmed.
More broadly, it's boorish and the height of bad taste to criticize an ally when it has been attacked. Trump only later sent his condolence.
And, both of these writers have rather smug and spotty memories. There was plenty of Obama Derangement Syndrome shown by conservatives during President Obama's term.