Morris's life and death as an allegory for futility
Morris is essentially a good man. In fact, just stepping into Morris's world causes Frank to consistently strive for a more moral life. And yet, Morris struggles and struggles for a breakthrough, and then when he finally gets what he's been hoping for, he robs himself of it by dying of pneumonia because he's too hasty and he forgets his coat.
This is the basic depiction of essentialism offered in the story. Basically, the idea is a dark humorous attempt to show that even if you do your absolute best and struggle toward a better life with virtue, you still might trip at the finish line and lose it all, underscoring the meaninglessness of the entire process. The universe does not return good behavior with a good life, but instead life seems to suck for everyone no matter what.
Helen's rape as an allegory for romance
Existentialism isn't exactly kind to the idea of romantic love, and this novel is no exception. When Frank meets Helen in the park, he finds he resisting a rapist, and he saves her. Then he rapes her.
There's a way in which this image shows the sad futility of love. Frank is not shown to be an evil man, but merely a man corrupted by his selfishness, trying to establish some integrity and character in a world that he wasn't built for. Part of this means that he wants to be a successful husband, but just as Morris trips at the finish line, so does Frank. This terrible scene is a compelling depiction of what happens in dysfunctional relationships, when two people volunteer for love, but then one or both of them ends up using and abusing the other person.
The store as a symbol for work
When Frank wants work, he finds Morris and asks him to work in the store. When Frank graduates to Morris's position after Morris dies, it's a statement of his ability to function in the world as an adjusted person, and the store comes to represent his ability to earn money and be self-sufficient.
Arson as a symbol for masochism
Masochism is when a person tries to find happiness by harming their self or their life. By trying to burn his own store down for the insurance money, Morris is figuratively abandoning hope for his success, and he's trying to find some sort of escape plan, or safety net, but he almost dies. This is an interesting depiction of masochism, showing that self-destructive tendencies can be born from the desire to do well for yourself. This is a common theme in existentialist literature and philosophy.
The motif of unconsciousness
Look at how many times a character is knocked out, either by attack, or by accident. This is especially true for Morris who finds himself constantly concussed—once by Frank when Frank first robbed the store, then again when he tripped moving crates, then again when he forgets to light the radiator, then again from his pneumonia.
This motif seems to underscore the blunt trauma of life and the disorienting nature of suffering. Frank's own plight could be understood that way, and his story would therefore be one of trying to reorient himself after trauma removed his ability to function as a successful member of society.
