What would Henry have said if he had followed his impulse to break away and " harangue " his fellow soldiers? Why does he stifle his impulses?

Henry would have warned everyone that he believed they were going to be slaughtered. He didn't.... because he might have been wrong.

As he looked the youth gripped his outcry at his throat. He saw that even if the men were tottering with fear they would laugh at his warning. They would jeer him, and, if practicable, pelt him with missiles. Admitting that he might be wrong, a frenzied declamation of the kind would turn him into a worm.