It is not fear mongering, it is a realistic and serious concern.
Even a thoughtful and intelligent president such a Kennedy brought us to the brink. Averting disaster in the nuclear age is a demanding task, even for a good president.
And yet you say we should not be concerned about a man who:
1. Has uncontrollable rage.
2. Has no empathy.
3. Has an all consuming ego.
4. Has said: "if we have them why can't we use them?"
There are very good reasons to be very concerned.