It appears that what really angered President Trump is less Waltz's incompetence (or worse) in keeping sensitive military communications secure, but rather his taking an active role in doing the bidding of a foreign government.

As the Post reported, in advance of Israeli Prime Minister Netanyahu's February visit to the United States the US National Security Advisor to President Trump…

…appeared to have engaged in intense coordination with Netanyahu about military options against Iran ahead of an Oval Office meeting between the Israeli leader and Trump, the two people said.

That means that Mike Waltz was working with a foreign government to maneuver President Trump into a situation where war seemed the only option left to deal with Iran. That kind of manipulation is a classic neocon move and one that Waltz's ideological allies managed with great success against President George W. Bush regarding Iraq.

According to one insider quoted in the article, Waltz, "wanted to take U.S. policy in a direction Trump wasn't comfortable with because the US hadn't attempted a diplomatic solution."

That means the former NSA was working with a foreign leader to limit the diplomatic and military options his boss could choose from, i.e. he was working to hobble the United States so as to achieve an objective of a foreign regime.

The WaPo piece continues…

'If Jim Baker was doing a side deal with the Saudis to subvert George H.W. Bush, you'd be fired,' a Trump adviser said, referring to Bush's secretary of state. 'You can't do that. You work for the president of your country, not a president of another country.'