So, he's either a moron or he's doing this because of some shady deal he has with Israel and Russia. I don't know what's worse though
This post makes no sense. Russia is one of the biggest supporters of the 2015 Iran deal and is the closest thing Iran has to an ally that isn't a puppet like Assad or Nasrallah. Russia is the last country that would engage in a 'shady deal' to kill a deal they pushed for and to reimpose sanctions on Iran.
Trump wants to not only put a stop to Iran's nuclear program for good, he also wants stop Iran's ballistic missile program, it's funding for terrorists like the Houthis and Hezbollah, and get Iran out of Syria. All of those things become much easier when he can bring the full weight of US sanctions against Iran's economy, and reimposing the sanctions now is easier than reimposing the sanctions when the nuclear deal's restrictions expire in the coming years. That's what Trump is thinking.