The circumstances under which Flynn "lied" to VP Mike Pence remain unclear. Regardless, for this Flynn was fired by Trump and Flynn's downward spiral began. Flynn was quick to find out that once one is the target of a special investigation - unless one is quite wealthy - the costs of defending oneself are staggering. Flynn soon realized that he faced millions of dollars in additional legal fees in the Mueller investigation (something AG Jeff Sessions never should have let happen) so Flynn copped a plea to lying to the FBI - regarding a trivial and legal activity in a FBI interview in which Flynn foolishly didn't have an attorney at his side. Is Mueller abusing the legal process? Not technically - but knowing that being targeted by Mueller means millions in legal fees or bankruptcy, the answer is a clear yes.