Deputy Attorney General threatened to quit for being made the scapegoat for Comey's firing

According to the Washington Post, Deputy Attorney General Rod Rosenstein threatened to quit his post because of the fact that the Trump administration had made him the scapegoat for the firing of former FBI Director James Comey, when in reality it was Trump the one who ordered Rosenstein and Jeff Sessions to write up an excuse to have him fired. From the WP article:

Rosenstein threatened to resign after the narrative emerging from the White House on Tuesday evening cast him as a prime mover of the decision to fire Comey and that the president acted only on his recommendation, said the person close to the White House, who spoke on the condition of anonymity because of the sensitivity of the matter.

A White House source told the WP Trump made de decision of firing Comey last Monday. The same day he ordered Sessions and Rosenstein to come up with the excuse to fire him. But Trump then claimed he was firing Comey because Rosenstein had recomended Trump to do so, which was not the case.

The real reason why Trump fired Comey, according to the WP article, was because of Comey's investigation into the Trump campaign's links with Russia in 2016.

Additionally, Trump was angry at Comey for not backing up Trump's bullshit story about Barack Obama spying on Trump:

Trump was angry that Comey would not support his baseless claim that President Barack Obama had his campaign offices wiretapped. Trump was frustrated when Comey revealed in Senate testimony the breadth of the counterintelligence investigation into Russia’s effort to sway the 2016 U.S. presidential election. And he fumed that Comey was giving too much attention to the Russia probe and not enough to investigating leaks to journalists.

So what got Comey fired was Russia and not going along with Trump's lies. Which is pretty much the same thing.