Wright's consistent thesis throughout his writing is that society is responsible for the monsters they piece together.
Dave is emasculated and disempowered by everyone he encounters: his mother, his father, his employers, his peers, and especially the society that shows him what can be had, but keeps the spoils just out of reach.
In an capitalistic society, Dave works and earns a paycheck, but he must turn over his earnings to his mother, who gives them to his father. He wheedles his mother into giving him enough money to buy a gun, a symbol of power, but power for which he is not mature enough to handle. Accidentally, he kills Jenny, the mule. When the truth of Jenny's death is revealed, everyone laughs at Dave. His boss, Mr. Hawkins, guffaws that Dave has "done bought a dead mule."
This line is a clever play on words, for after emancipation, every freed slave was entitled to "forty acres and a mule." Freedom doesn't mean much when there is nothing to do with it (a dead mule.) Black men, Wright argues, have been systematically denied the ability to use their "freedom" to better themselves. Their disenfracisment turns into wild, uncontrolled anger. In such a society, men will always almost, but not quite, be men. Dave, therefore, is not to blame for his actions. He cannot be held accountable, for he has never counted for anything.