Reportedly Milley anticipated Trump attempting a coup and actively worked behind the scenes to make sure it wouldn't work.

https://www.cnn.com/2021/07/15/politics/milley-trump-politics-january-6/index.html

Milley was deeply concerned Trump and his allies might attempt a coup after the November 2020 election, and he compared Trump’s lies about election fraud to the rhetoric used by Adolf Hitler as he rose to power in Germany.

“This is a Reichstag moment,” Milley told his aides, according to the book. “The gospel of the Führer.”
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The official acknowledged that during the final weeks of the Trump administration, Milley engaged in activities and communications not part of the traditional portfolio of a Joint Chiefs chairman, carrying a heavier political load to keep Trump in check. While Milley “tried his hardest to actively stay out of politics,” the official said, if the events that occurred brought him into that arena temporarily, “so be it.”

“He’s not going to sit in silence while people try to use the military against Americans,” the official said.
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After the election, Trump’s blustery rhetoric turned into action when he fired Esper and installed loyalists at the Pentagon, raising Milley’s fears that Trump or his allies could try to attempt a coup to overturn the election results.

According to Leonnig and Rucker, Milley was shaken by the threat of a coup and felt he had to be “on guard” for what might come.

“They may try, but they’re not going to f**king succeed,” Milley told his deputies, according to the authors. “You can’t do this without the military. You can’t do this without the CIA and the FBI. We’re the guys with the guns.”

Milley and the rest of the Joint Chiefs informally planned for what they would do in the event of an order they deemed illegal, dangerous or ill-advised, including a proposal to resign, one-by-one, rather than carry out the orders.

After the January 6 attack, Milley and the entire Joint Chiefs of Staff issued a statement condemning the “sedition and insurrection” of the rioters who breached the Capitol and attacked police officers.

Milley focused his efforts on ensuring there was a “ring of steel” around the city for the January 20 inauguration of President Joe Biden, according to Leonnig and Rucker. “We’re going to put a ring of steel around this city and the Nazis aren’t getting in,” Milley said.

They hate him because he lived up to his oath to defend the Constitution against all enemies, foreign and domestic. They hate him because he's an actual patriot.