Do you or don't you believe in the power of the monkey's paw? This is one of those stories, unlike the Harry Potter books, where we aren't ever quite sure whether supernatural forces are at work or not. It's like "The Monkey's Paw" is trying to convince us of two conflicting things: 1) that the paw really is magic and directly causes Herbert White's death; and 2) that the paw isn't magic and Herbert's death just happens to coincide with Mr. White's wish for two hundred pounds.
But which is it? If you're feeling confused and uncertain, you're not alone. This sense of confusion is exactly what the characters are feeling throughout the story. Even the very rational Herbert White, who tries hard to show he doesn't believe in the paw's powers, sees (or thinks he sees) the face of the evil monkey in the fire. Rest assured that W.W. Jacobs wants us to be unsure, and that's a big part of what makes this story so eerie and so awesome.
The paw is fueled by the monkey's anger and vengeance, which makes it impossible to wish on it without negative consequences.
The story's ending proves that the paw's powers are real.