What does killing a man have to do with Pepe and his construction of "manhood?"
I think that Steinbeck makes it clear that Pepe's notion of "being a man" is so skewed and far off from what it really means that believing in killing a man would fit into such a construction. Pepe is so lost that he actually believes that when he leaves, "he is a man." There is little in his characterization that reflects he actually knows what it means to be an adult, or that he grasps the struggle that is involved in being on his own. It is for this reason that Pepe so quickly clings to the idea of "being a man." Steinbeck seems to be suggesting that modern constructions of "manhood" and masculinity are shrouded in what one thinks these ideas are as opposed to what they actually entail. In Steinbeck's development of Pepe, it would make sense that something so small and so contingent would be the cause of an action so profound. Someone says something to Pepe and he reacts with a weapon. Steinbeck might be suggesting that the culture of violence, one in which "manhood" is defined through violent action, is what actually helps to kill Pepe. In believing that "a man" is prone to violence, Pepe actually becomes a victim to it, himself. In this, Steinbeck is both criticizing Pepe and the social values that define "manhood" and masculinity as realms in which violence and murder are so closely associated with them. Pepe is victim to this, sure enough as his victim succumbs to the weapon.