Jesus used the well-known Hinnom valley fires (and not some unfamiliar place accessible only after death) as an analogy of the 'hell' fire to be used to kill (not torture!) forever those who refuse to repent of hatred, anger and open condemnation of another person:
"And whoever says to his brother, 'RACA!' shall be in danger of the council. But whoever says, 'YOU FOOL!' shall be in danger of hell fire." (Matthew 5:22)
The literal meaning of the Greek word Gehenna (Strong's Concordance #G1067), translated as "hell" in Jesus' statement, is the 'valley of Hinnom.' Other places where Gehenna is erroneously translated as "hell" include Matthew 18:9, 23:33; Mark 9:43 and others. The first mention in the Bible of the valley is in the book of Joshua where the borders of the tribe of Judah are defined:
"And the border went up by the Valley of the Son of Hinnom to the southern slope of the Jebusite city (which is Jerusalem). The border went up to the top of the mountain that lies before the Valley of Hinnom westward . . ." (Joshua 15:8)
Hinnom is a deep, narrow ravine on the south end of Jerusalem. Anciently (before the time of Jesus), it was at this location that idolatrous Israelites worshipped the pagan gods Moloch (Molech) and Baal using, among other things, fire. Many of the kings of Judah (and Israel), like Ahaz (who ruled beginning in 735 B.C.), personally endorsed the worship of false gods using fire. Ahaz not only made idols representing false gods, he sacrificed his own sons by having them BURNED and offered as a burnt offering to his pagan god (2Chronicles 28:1 - 3)!
One of the reforms of Josiah, who reigned over the Kingdom of Judah from 640 to 609 B.C., was to destroy those places used to worship false gods. The Bible states one of the practices stopped by Josiah was the following.
"And he defiled Topheth, which is in the Valley of the Son of Hinnom, that no man might make his son or his daughter PASS THROUGH THE FIRE to Molech." (2Kings 23:10)
What does it mean to 'pass through the fire?' Clark's Commentary states that toph, from which the valley of Topheth gets its name, means 'drum.' The valley was given this name since drums were used to drown out the cries and screams of CHILDREN placed on the red-hot arms of Molech ('passed through the fire') to burn to death as a sacrifice to this evil pagan deity!
At the time of Jesus, Hinnom valley was not used to worship false gods. Fire however, because of its long tradition of use, continued in the place albeit with a still unpleasant purpose.
"After the return of the Jews from captivity, this place was held in such abhorrence that, by the example of Josiah, it was made the place where to throw all the dead carcasses and filth of the city, and was not infrequently the place of public executions." (Albert Barnes' Notes on the Bible on Matthew 5:22)
"a fire was kept burning in it to consume the carrion and all kinds of impurities that collected about the capital. " (Jamieson, Fausset and Brown Commentary on Matthew 5:22)
Some commentaries have also suggested that Judas, after he betrayed Jesus, used a tree near a precipice overlooking the Hinnom valley ('hell') from which to hang himself.