Although these two stories seem completely different at first, one can see that there is a similarity in their themes. In a sense, both are stories about how cruel fate can be.
In "The Monkey's Paw," fate gives the White family what they ask for. However, it gives it to them in a horrible way, by killing their son and then (it is implied) bringing him back as some sort of zombie. Similarly, the title character in the sniper gets what he wants. He manages to kill his opponent. But here, too, he is given what he wants in a horrible way since the opponent turns out to be his brother.
In both stories, characters get what they want, but fate is so cruel that they might well wish they hadn't. This is the major similarity between the two.