In the opening of 28 Weeks Later, Don (Robert Carlyle) faces a dilemma: He can leave his wife to die and run like hell on the off chance that he might outrun the “infected,” or he can stay with her and face a gruesome end.
He runs like hell, and looks back to see his wife attacked.
This is the movie writ small, laying the groundwork for more impossible choices.

The Psychopathic Chicken (and Other Lessons of Evolution)