Why did Trump put a former foreign agent in a key security post?

It's been nearly a month since Michael Flynn was forced to resign as White House National Security Advisor, following revelations that he lied about his communications with Russia. Flynn's controversies, however, are not yet over. The Associated Press reported:

President Donald Trump's former national security adviser, Michael Flynn, who was fired from his prominent White House job last month, has registered with the Justice Department as a foreign agent for $530,000 worth of lobbying work before Election Day that may have aided the Turkish government.Paperwork filed Tuesday with the Justice Department's Foreign Agent Registration Unit said Flynn and his firm were voluntarily registering for lobbying from August through November that "could be construed to have principally benefited the Republic of Turkey."