Nick sees something that suddenly makes him leaves the car and seek Gus is a state of high anxiety.
What does he see?
I have examined the building and do not see the reason for his behaviour
It's not something he saw, it's something he put together / thought of in the car. He figured that the location was actually a trap. But he figured out too late and couldn't save Gus in time.
shareHe read his partner would be killed in the elevator as the book was being printed off the computer.
shareHe remembered what he read in Catherine's novel.
shareThis confused me for years. If you pause the scene when Catherine breaks it off with him when the paper is printing, it says that Shooter raced into the building, partners body lay dead, elevator, etc.
He puts that together along with the fact that Catherine's books always come true. The ending reveal is a big one, but it's not surprising or a twist. Catherine's character told us all along that was going to happen.