Iran has the right to a peaceful nuclear energy program under the NPT. However, multiple facilities that have come to light recently have no purpose outside of a military program, which would be banned under the NPT, and other UNSC Resolutions.
Additionally, there are numerous UNSC Resolutions that express call for the halt of uranium enrichment, so even enrichment for a peaceful program would be in violation of international law at this point.
One bomb will of course hardly "kill everyone."
However, a nuclear Iran will set off a wave of proliferation throughout the Middle East, resulting in much more fissile material in the region that some rouge group might get their hands on. Additionally, to be frank, I would be more concerned about a Saudi nuclear weapon than an Iranian nuclear weapon, and rest assured, once Iran goes nuclear, Saudi Arabia will not be far behind.