Sharply criticizing Trump's remarks on tariffs, Trudeau promised that Canada would answer with its own on July 1 unless the USA reversed course.
On Sunday, Trudeau the younger demurred even more, choosing instead to send out messages of his own about democracy, women's empowerment and the environment.
France and Germany both reacted with anger to Mr Trump's decision.
G7 leaders were hoping for a summit that presented worldwide unity on a number of global issues.
Trump's closest advisers added to the attack during televised interviews on Sunday.
On Sunday, a day after Trump refused to sign a communiqué of the Group of 7 major industrial economies, his advisers went on the attack, accusing Prime Minister Trudeau of "betrayal" and a "stab" in the back, even as Canada, Germany and France pushed back against what they called the United States president's "insult" and "inconsistency".
However, Canadian Prime Minister Justin Trudeau later said in a press conference after Trump left the summit that retaliatory tariffs against the USA would still be implemented on Jul. 1. Trump is due to meet with North Korea's Kim on Tuesday.
Then he claimed Trudeau was "pouring collateral damage on this whole Korean trip". Trudeau made an error.
Kudlow suggested Trump saw Trudeau as trying to weaken his hand before that meeting, saying the president won't "let a Canadian prime minister push him around". At one point he wrote, "Justin acts hurt when called out!".
He warned that the USA will take any action necessary to defend Americans from unfair trade practices.
Trump took aim at an unspecified news release issued by Canada saying, "they made nearly 100 bln dollars on trade with u.s. (guess they were bragging and got caught!)". "Our Tariffs are in response to his of 270% on dairy!" he said.
Trudeau hosted the summit on the back of several tense weeks of trade discussions with the United States, who imposed tariffs on aluminum and steel imports from several countries a few months back, but offered Canada and Mexico an exemption from the fees as they continued NAFTA discussions.
German Foreign Minister Heiko Maas said: "In a matter of seconds, you can destroy trust with 280 Twitter characters".
Trudeau's office responded to Trump's tweets with a written statement, saying the prime minister's comments in public and in private with Trump were "nothing he hasn't said before" and that he was "focused on everything we accomplished here at the G-7 summit".
He described Trudeau's actions after the G7 summit as a "sophomoric play".
Trump trade adviser Peter Navarro said: 'There's a special place in hell for any foreign leader that engages in bad faith diplomacy with President Donald J. Trump and then tries to stab him in the back on the way out the door'.
"It is a historic negotiation and there is no way this president is not going to stand strong", Kudkow said on CBS' Face the Nation.
"They should have said to him, 'God speed, you are negotiating with the insane nuclear tyrant in North Korea, and we are behind you, '" Kudlow added. "The collapse of USA governance before our eyes".
Former FBI Director James Comey also weighed in, tweeting Saturday night on the US-Canadian relationship. He had other things, bigger things, on his plate in Singapore.
The president has accused Canada of taking advantage of American workers through their trade practices.
"We commit to modernize the WTO to make it more fair as soon as possible".