General Mattis is an exceptional man. As an officer leading men into battle, he naturally has made statements that, out of context, seem crazy.
General Mattis is very, very widely respected within the military. In fact, if America were going to become a dictatorship, General Mattis is the kind of man you need supporting you to succeed.
This is precisely the problem: because of his power and credibility within the military, Mattis' association with Trump scares the Establishment: Mattis protects Trump from any undermining within the defense community and also neutralizes a lot of opposition from within the intelligence community.
It is important to realize that the CIA, today, is full of leftists. Yes, you read that right: people who sincerely believe in "white privilege" and that Obama was "chosen by the people" rather than shoe-horned into American politics by cunning oligarchs. In fact, the director of the CIA, John Brennan, voted for the Communist Party in 1976.
These Leftists within the CIA loathe Trump and his supporters, and they do not consider it treason to undermine his authority, even at the cost of eroding what little civic decency we have left in this country. There is a fight within the intelligence community, and they realize that under a potent Trump administration, their campaign of unchecked power ends.
The MSM is collaborating with this element of the CIA and the oligarchs who support them to undermine Trump's key guard-dog, General Mattis. This is why they are trying to make him look like a lunatic and either scare him away from office or sabotage his career before he even starts it (it won't work, but they will still try).
Ending the unchecked power of the left in the CIA would indeed make America great again.