Why does the West fear Iran is secretly trying to build atom bombs?

I don't trust the leaders of Iran to not use a nuclear weapon on Israel. There is a hatred there of the Jews that goes beyond rational thinking that many don't understand. The situation is different than when the US and USSR faced off. during the Cuban Missile Crisis one many was poised with orders to fire at US warships. He didn't because he understood exactly what was at stake, the world. I fear that the leaders of Iran do not understand that if nuclear war breaks out, there is no winner there for they will show no restraint in using such a weapon.