While there are several suspenseful moments, the main climax is the point at which the main character changes his point of view. Usually, this moment comes near the end of the story. Thus, in "The Monkey's Paw," the climax occurs when Mrs. White asks her husband to wish Herbert able to return to them because Mr. White changes his perspective about the monkey's paw, fearing it now. He asks his wife,
"If he was too terrible for you to see then [after the accident], how now?"
Mr. White goes to the parlor and finds the talisman;
...a horrible fear that the unspoken wish might bring his mutilated son before him ere he could escape from the room seized upon him.
It is with terror that Mr. White retrieves the paw and brings it back to his and his wife's bedroom where she orders him to "Wish!" But, Mr. White fears that Herbert is, indeed, returning, but in the mutilated form in which he died.
"Don't let it in," cried the old man, trembling.
Then, he gropes desperately for the paw so that he can make the third wish before the Herbert enters.
I would say it is when Herbert is killed in a factory accident after Mr. White wishes for two hundred pounds. All of the mystery surrounding the monkey's paw, and the warnings about not using it, lead up to Mr. White ignoring the warnings and making a wish, and the death is the immediate consequence for this action. While the implication of a zombie Herbert is exciting and weird, I feel like it would fall after the climax, as it is a direct result of the tragic death as opposed to the culminated result of all previous events.