Trump pulled out of the Paris accords. A voluntary agreement with no penalties for missing targets. He used a raft of misleading and incorrect statements (full link here) to justify an unjustifiable resolution. He did it to say fuck you to President Obama and the world who thought they could deal with him. He did it against he advice of pretty much ever CEO, except a small amount of coal producers - because it will hurt the US economy.
He did it to show he can do what he wants.
There is no point in argue the merits of this, there are no merits in pulling out of a voluntary agreement that was worked out based on US requirements.
No one should be surprised. He promised to do this repeatedly on the campaign trail and he has.