Strictly speaking, he said that bad things you're talking about would happen "if Ukraine joins NATO and tries to retake Crimea by military means." Not, as you seem to imply, just for the fun of it.

The U.S. has also threatened, or considered first-strike nuclear attacks several times (albeit not publicly) numerous times, as recently as 1997. Usually against smaller countries (the usual suspects) but also against the Russians in the event of an attack on Berlin during the 1961 crisis:

http://www.theatlantic.com/magazine/archive/2001/10/jfks-fir...

So say what you want about the petulant madman - but really he was just pulling a move from Kennedy's playbook, essentially.