Why is Johnny sick to his stomach when Rab leaves? From chapters 9-10

Rab gets ready to sneak out of Boston. He wants to go to Lexington and join the Minute Men there, but he knows that once the fighting starts he won’t be able to get out of Boston. Johnny is resentful, both because Rab doesn’t seem to want Johnny to come along, and because Rab is willfully ignoring the danger he will face. Despite Johnny’s protests, Rab goes merrily on his way. All Johnny can think about is the horrible sight of the musket barrels at Pumpkin’s execution.