Would Iran actually build a nuclear weapon? That is an intriguing question, but there is no determinate answer.
Nuclear proliferation is an inescapable fact. Many nations have joined the nuclear club started by the United States in 1945. Nuclear weapons are abhorred by most people, and some argue that they should be abolished. This seems impractical, though. Although the weapons could be destroyed, mankind's knowledge of how to construct them cannot be erased.
A Iranian bomb would pose a serious threat to Israel. As we enter the third decade of the twenty-first century, Iran is Israel's most dangerous enemy. Iran and many other nations in the Middle East claim they have a right to build nuclear weapons because Israel already has at least one hundred of them. If it felt threatened, Israel would probably launch a preemptive strike against Iran—as it did against Iraq in 1981.
Saudi Arabia would also be threatened by an Iranian nuclear bomb. Saudi Arabia and Iran are bitter rivals and they lead opposing Muslims factions. If Iran developed nuclear weapons, Riyadh would almost certainly obtain them, too. In fact, Pakistan—a close Saudi ally—would probably assist Riyadh in their development or even give a couple to Saudi Arabia.
Nation states, including Iran, are extremely reluctant to use nuclear weapons. If Iran were to attack Israel with one, the U.S. might respond in kind, and Iran would cease to exist. Terrorists would use nuclear weapons if they had the opportunity, however.