As long as Iran are under the agreement, their "breakout time" to create a nuclear weapon is estimated to be more than a year. Without it, it's a few months.
Even if Trump (or rather his administration, as Trump likely doesn't understand much of this) think the inspections provisions are inadequate, which according to supporters are considered "the most rigorous ever brokered", leaving Iran to their own devices free to develop nuclear weapons at their leisure with no inspections, and the only hurdle being sanctions, is a terrible idea. Mainly because Iran already deals with many other countries in Europe, including Russia.
The funds were already Iran's to begin with. Their assets were just frozen. It did not come out of USA's pocket.
And the "Israeli intelligence documents" Trump referred to were already known to USA before this deal was brokered. They apply to Iran years ago. Not during the agreement. So citing them as motivation for this action without revealing that crucial piece of information is nothing more than a dishonest way of swaying public favor.