Based on real life events, Assassination is set in 1974 and centers on a businessman who decides to take extreme measures to achieve his American dream.
If you've watched this movie, Sean Penn plays a fragile salesman who questions his morals and starts to believe that the government is racist because they won't let him start his business because his partner is black.
He ends up quitting his job (which he thought he was about to lose it anyway?) and gets divorce papers from his wife. He blames all of it to the current president.
He finally decides to do the plane hijack (and fly it to the White House?) all by himself. He prepares for it (kind of) and takes the partners gun from the shop.
Then he goes to the restaurant planning to kill his ex-salesman boss, but he doesn't manage to.
And what he does next, he goes to his ex-wifes house and there's nothing but their dog. And he loved that dog and tells to himself that the dog loves him, then he brings that dog to the basement and shoots it.
Why did he shoot the dog? Did he feel he needed to train shooting before hijack? Or is there deeper meaning?