So what are the chances that the Madman's voiced concerns about Flynn were mere altruism?

[The Velveteen Ocelot]

7. Exactly. Trump never hesitates to throw people under the bus

when he no longer needs them or if they displease him. The only reasonable explanation for his almost desperate defense of Flynn is that he doesn't want Flynn to talk. And whatever it is that Flynn knows must be something spectacularly bad. He had to fire him for the sake of appearances, but doubtless he told Flynn he'd do whatever he could to protect him from legal and political consequences as long as Flynn promised to keep his mouth shut.