Iran already has strained relations with many countries. Closing the strait would ensure a global coalition against them, maybe even getting Russia to denounce them as well.
Why? Russia would love the price of oil to spike. Russia sells lots of oil.
Most counties would denounce Iran for closing the straights. But the US has forced most countries to do this already. I mean Europe wanted to keep the Iran deal going, but the US basically extorted European companies into obeying them anyway. Most countries are not going to go to war over it. And most countries have been forced to take part in the sanctions anyway. Most countries can't really do anything about it.
But the US has forced their hand. The murder of the most popular government official in Iran requires them to react. That means terrorist attacks, kidnappings/murders, or closing the straights. I'm sure that lots of people will blame Iran when this happens, but Trump made it impossible for them to do anything else. When one or more of these things happen, that blood will be on trump's hands.