Some believe that Jesus refers to sinners as worms and says that those people would never die but would live on in agonizing torment. Those who say this fail to notice that Jesus does not call wicked people "worms," but speaks of "their worm." The original Greek word for worm means "grub" or "maggot."
Jesus refers to a local method of garbage disposal to emphasize the permanent consequences of unrepented sins. The margins of some Bibles show that the words "hell fire" in Mark 9:47 should be translated "Gehenna fire." Gehenna, or the valley of Hinnom, is located outside Jerusalem. Trash, refuse, animal carcasses, and even the dead bodies of despised criminals were thrown there to be destroyed by the fires that burned perpetually on the valley floor. If some animal or vegetable matter fell on one of the ledges below the rim, escaping the fire, it would instead be devoured by maggots.
Jesus' point is that whatever was thrown into the valley never came out again; it was totally consumed, either by fire or by worms or maggots. In other words, just as nothing and no one exterminated the maggots or extinguished the flames in the valley of Gehenna, so there will be no escape from the certain fate that God has decreed for all unrepentant sinners—death in the "lake of fire" (Revelation 20:14-15).
Some of these same principles apply to Isaiah 66:24. The meaning is not that unrepentant sinners or worms live forever. In fact, the opposite is the point of the passage. If the worms that infest a dead body are not killed, the rotting flesh will be consumed until none remains. Maggots, which are simply larval flies, go through a process known as pupation and turn into adult flies. These, in turn, deposit additional eggs, and the process is repeated until nothing is left for maggots to feed on. Similarly, any fire which is not quenched—not deliberately put out—will last only as long as there is fuel to keep it burning and then go out. The whole point is that, when a person does not repent of sin, the results are absolute and permanent—eternal oblivion!