Before I talk McMaster let's talk Flynn. Barely on the job he quit, actually a casualty. For doing what Trump ordered him to do. Which is to sound out Russia via its embassy but he did it before he became NSA. He is therefore the perfect fall guy for the Neocon gang who runs the Deep State. As the entire world knows, the American Deep State is all about war and ideological struggle with Russia and China. Any of Trump's people who dares to talk to the Russian is automatically a traitor to the Deep State and will be gone. 

This incident confirms that the Deep State not only control U.S. foreign, military and intelligence policies, they have all the apparatus to run these policies. They are powerful beyond the president. 

Now McMaster. Why did he agreed to the job? Participate in virtual civil war between elected president and un-elected but all powerful Deep State. There are only two reasons. 1) He is ambitious and love a fight for Trump. Or 2) he is a mole planted by the Deep State.