Of course O'Reilly should defend this soldier and do it aggressively. Only rabid anti-Americans and extremist liberals, like Chrissy Matthews, would do otherwise.
Those not there have no legitimate opinion on the matter, and combat veterans, while they do, should allow for the dangers of the specific situation to be considered mitigating.
No one can judge that man a murderer when he killed an enemy combatant in a war zone who was pretending to be dead.