The theme of "Paul's Case" is that when the world of illusion, of the superficial, becomes too appealing to us, it can destroy us.
We can feel some sympathy for Paul as he rejects the narrow, hard-working, respectable, and Calvinist world he has grown up in. He wants more than a drab life working in an office and sitting on a Pittsburgh stoop in the evenings. He longs for beauty, but his longing is entirely shallow. We are told that:
It would be difficult to put it strongly enough how convincingly the stage entrance of that theatre was for Paul the actual portal of Romance.
In other words, Paul can't distinguish between surface illusion and reality.
Paul becomes so desperate for a more colorful, beautiful life that he steals a thousand dollars from the bank where he works, quite an amount of money at the time, and heads to live the high life in New York City. This life completely appeals to him. The story says he feels no remorse:
The flowers, the white linen, the many-colored wine glasses, the gay toilettes of the women, the low popping of corks, the undulating repetitions of the "Blue Danube" from the orchestra, all flooded Paul's dream with bewildering radiance. When the rosy tinge of his champagne was added—that cold, precious, bubbling stuff that creamed and foamed in his glass—Paul wondered that there were honest men in the world at all. This was what all the world was fighting for, he reflected; this was what all the struggle was about.
Because he is so shallow, this is all Paul needs. Everything for him is surface. He doesn't desire to accomplish anything. He simply wants to live in a certain way:
He felt now that his surroundings explained him . . . He had only to glance down at his attire to reassure himself that here it would be impossible for anyone to humiliate him.
For someone for whom the window-dressing is everything, we now get a glimpse of how humiliating it was for Paul not to have beautiful things.
Paul kills himself rather than face up to the consequences of his actions. The story leaves us both wondering at his shallowness—is this what his society has told him is important? Is he merely a sociopath?—and wondering if, since the superficial is all he ever wanted, he achieved his dream.