The only real case is that Iran has a history of sponsoring various terrorist groups around the region and the world. They also provide them with pretty sophisticated arms (Drones, shaped munitions, MRBM's, stuff like that). Were Iran to get a functioning nuclear weapon manufacturing ability, it is *very* likely that one of those weapons would find it's way into the hands of a terrorist group somewhere. Note: not the first bomb, or even one of the first ten, but if they have the ability to make weapons on demand, those weapons generally find their way to Muslim terror groups. Iran is pretty open about using Jihad by small non-state actors as their method of spreading Islam, and exacting revenge for slights (real or perceived).
Whether that is enough of a case for you is up to you.
