Let's see...
Iran backs the Houthi who have tried to start a civil war with Saudi Arabia and overthrow that nation's government. They were supplied missiles and attacked shipping in the Red Sea to try and close down that waterway, all with Iranian backing.
Iran supplies arms and munitions to all sorts of Islamic terrorist groups all over the Middle East.
Iran has tried to overthrow the new, post Saddam, government of Iraq using Shiite proxy forces within that country.
Iran and Pakistan (the latter being nuclear armed) have cooperated closely on military arms production and sales.
Iran has on repeated occasions unilaterally closed the Strait of Hormuz claiming sole sovereignty over it even as Oman has equal political and legal standing.
Iran is a huge destabilizing influence in the region.