Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is an intelligent horror/sci-fi made by George A. Romero about a biological weapon made by the U.S. military that has accidentally spread through a small Pennsylvania community. It has a brief snake cameo near the end.
Soldiers in white suits have been rounding people up, killing those who appear to be infected with the virus which makes them act crazy. A group of people try to fight back and escape from what appears to be a mass execution of the town's residents. Clank (Harold Wayne Jones) who is probably infected, separates from the group to distract the soldiers from the others so they can escape. As he runs through the woods, shooting at the soldiers, he stops to take cover behind an old fallen tree. He sees a snake crawling on the tree and stops to watch it for a minute. Then he starts shooting again as the soldiers close in on him.
The snake is a rat snake, the kind that can be found in a Pennsylvania woods. I don't know why the movie stopped to show it. I don't think it's there to scare us, and Clank doesn't kill it or take it as a threat. Since it's a movie and in movies tough guys seem to kill every snake they see because all snakes are dangerous, maybe what it's showing us is that he didn't bother to kill it because there is a far worse danger and because he knows he's going to die soon.