When did he say that?  In fact what he said was they'd be met with fire and fury if they continued to threaten the US. (“North Korea best not make any more threats to the United States.” “They will be met with fire and fury and frankly power, the likes of which this world has never seen before,”  ).  The fact he hasn't yet acted on these threats by getting the UN to continue imposing stricter sanctions shows reasonableness and not recklessness as you would seem to counsel.  He's also showing that he doesn't have much patience and I'm sure his remarks encouraged China and Russia to implement those same sanctions knowing full well that if they didn't sign up for them, Trump might very well implement the fire and fury.