There are important observations that a reader can draw, but which the novel doesn't point to explicitly. Firstly, Hermann blatantly ignores his wife's incestuous affair. That's an indication of an unbalanced person. Then the reader could not that the murder seems to project his own image onto an unsuspecting person, but most of all, the reader could notice that Hermann feels comfortable killing his own spitting image.
The point of that comfort is that it indicates that Hermann is unhinged from himself. His life is far from what it seems on the surface. On the surface, he's a happy candy maker with a loving wife. Underneath, there's a man who has been humiliated by his wife, standing in a psyche that is no doubt insane. The pressure makes him crack, and his murder has the effect of a quasi-suicide.
Ultimately, the police have the final word. When the police showed up to collect the body, they saw that the dead man was most certainly not Hermann. Apparently Hermann's perception was flawed the whole time. The effect of the novel is that the unsureness about one's own perception, coupled with unique suffering, leads to self-hatred, which in turn leads to murderous rage. The process is perhaps best summed up in the novel's namesake: Human despair.