You'll annoying about the ending.(spoilers alert)
The author did not entertain as you wanted. He wanted to show us what's real life and what can happen in real life. The point is that if this is a real life, will you give up easily or quit from life because you're afraid of suffering. We all know that most of us are suffering from bad jobs, neighbors, fake friends and many other else. The main point is that the monsters are the depression and you are struggling with depression, will you suicide because of the depression? The author make a very unique movie with an annoying ending just to show us what is real life monsters and suffering. If he want to be famous, he can easily make very different happy ending. But he choose not to follow the famous and earnings. He chooses to message us something.