This story is a modernist story, and one of the traits of the style of modernism is that the characters contained in the story are often alienated and disconnected, and have a hard time communicating effectively with one another. Dave, who resents being poor and resents being treated like a kid from the other slavehands (see the very opening passage of the story; Dave is complaining about the field hands teasing him like he's a kid), feels isolated and alone. He feels picked on, misunderstood, and not included. His mother seems to be pretty ineffectual in holding any sway over him, and he outright disobeys his father, who tends to be a bit overbearing. So, with no friends, no real connection with his parents, with constant badgering from those he works with, and with his poor station as a share-cropper, Dave doesn't really have much of a monetary living, and spends his emotional living being filled with angst, and finding ways to be noticed.
All of these forces combine to make Dave feel powerless. Sharecroppers never made enough money to get ahead, and he was part of that legacy. He worked another man's fields for almost nothing. Then, you add neighbors and coworkers who tease him, and parents that he is distant from, and it is the formula for a kid with some esteem and anger issues. This is why he gets the gun; he can earn respect, be noticed, and prove his manhood. If only he understood that manhood is making responsible decisions and paying for the consequences of your actions...but, he doesn't, and hence the title "The Man Who Was Almost a Man."