Why does Mr. Morris throw the monkey’s paw into the fire?

After Mr. White asks Sergeant-Major Morris about the monkey's paw he attained in India, Morris is reluctant to speak about it, then proceeds to tell the White family that its first owner's last wish was for death. After mentioning that he is not sure whether he would want three more wishes, he suddenly throws the magic monkey's paw into the fire. Sergeant-Major Morris understands the malevolent powers of the monkey's paw and is aware that it can wreak havoc on the White family, which he is why he attempts to destroy it by throwing it into the fire. Morris is a friend of the White family and does not wish to see them hurt by the monkey's paw. By throwing the magic monkey's paw into the fire, Morris is being proactive and protecting the White family from the evil talisman. Tragically, Mr. White retrieves the monkey's paw from the fire before it can be destroyed and proceeds to make his first wish after Morris leaves.

Sergeant-Major Morris knows what the monkey's paw can do; he knows what strange powers it holds. He has direct personal experience of this, and doesn't want anyone else to have to go through what he did. He explicitly tells the Whites that an old fakir, or Indian holy-man, put a magic spell on the paw to show that fate ruled people's lives and that interfering with it would only lead to sorrow.
Despite Morris's warning, the Whites don't seem convinced; they treat the monkey's paw like it's nothing but a big joke, a harmless piece of mumbo-jumbo. As the paw has already caused so much mischief, and as the Whites clearly don't take its magic powers seriously, Morris throws it onto the fire. Foolishly, Mr. White immediately swoops down and snatches the monkey's paw from the flames. Morris advises him to throw it back on the fire, but White chooses to keep it instead, with tragic consequences.