All appointed officials who work in the Executive Branch of the government, i.e. NSC, DOJ, FBI, CIA, State Department, etc. all work at the pleasure of the President. He can fire them anytime he wants with or without justification.
So far, everyone who has testified, with the exception of Vindermen, worked in an appointed position and is required to carry out the policies of the President. They don't get to decide if they like the position or policy.