While it is likely that Trump was being sarcastic and using this is a media grabbing tactic to go after HRC (I'm not a fan of either and not American - so my views aren't relevant.
This is, however, a historical precedent on why having the leader of a nation state make off the cuff remarks is dangerous to peace.
Consider way back in 1770 when Henry the II was getting annoyed at the Archbishop of Canterbury because he was doing things the king didn't really approve of, Henry uttered those infamous words, 'Who will rid me of this turbulent priest.' 4 knights overheard and long story short, one dead arch bishop.
The impact of this was huge - even by medieval standards.
Now imagine a modern day scenario where a president who has a well known temper, lashes out against those who 'oppose' him, and isn't known to staying on message.