Why does Ross feel he has to take his experiment even further?

The character of Ben Ross, based on a real-life teacher in California who conducted the experiment fictionalized in the book, is described as a man who easily becomes obsessed with his undertakings. Ross' devotion to his interests—namely Native Americans, the game of bridge, and Nazi Germany—gives him the reputation among other teachers as a genius bordering on insanity. However, Ross has full control of the experiment and makes sure all the children are psychologically and physically safe during the program.
When the behavior of the students begins to worry other teachers and school administrators, Ross decides to continue the experiment for one more day, not because he is obsessed with the program, but so he can reveal to the students how their mentality has changed over time. He reveals that there is no centralized leader in The Wave and that it had no principles or goals. Instead, he shows a picture of Hitler to demonstrate how the students began to think like the Nazi soldiers.