He is actively pre-empting attempts to derail his primary point.
It's possible he also believes that Awlaki committed treason, but more importantly, by making that point without any kind of moderation, he's making it substantially harder to accuse him of wanting to be soft on those horrible, evil terrorists.
He's also implicitly making the argument for a trial stronger: If it is entirely certain that Awlaki committed treason, then surely the government has clear evidence that would have made a trial a formality?
It's possible he also believes that Awlaki committed treason, but more importantly, by making that point without any kind of moderation, he's making it substantially harder to accuse him of wanting to be soft on those horrible, evil terrorists.
He's also implicitly making the argument for a trial stronger: If it is entirely certain that Awlaki committed treason, then surely the government has clear evidence that would have made a trial a formality?