During the 18th and 19th centuries, it came to be seen as unnatural to be able to watch someone executed, but that has never stopped some people from watching when given the opportunity, and it probably never will. Executions have always attracted people from all backgrounds: men, women, and children; rich and poor; academic and illiterate. Individual responses may differ—some will laugh and jeer, others will studiously take notes, some will faint or vomit or cry—and to an extent these responses are culturally determined, but the lesson of history is that it is within our capacity as humans to witness decapitations and other forms of execution and, more than that, to enjoy them as popular public events.
For as long as there were public executions, there were crowds to see them. In London in the early 19th century, there might have been 5,000 to watch a standard hanging, but crowds of up to 100,000 came to see a famous felon killed. The numbers hardly changed over the years. An estimated 20,000 watched Rainey Bethea hang in 1936, in what turned out to be the last public execution in the U.S.
— Adapted from Severed: A History of Heads Lost and Heads Found, by Frances Larson (published this month by Liveright)