Christians - I really don't understand why Hell is supposed to be a miserable fiery place of torment. Shouldn't just be filled with sin?

There are various reasons:

1. Hell is a place of punishment because justice demands that sins be punished.  Imagine, for instance, a rapist who took great pleasure in picking the most innocent and vulnerable victims - even including children - and who never repented. Would it be just for him to escape with no punishment either in this life or in the life to come?

2. In hell, people will have cut themselves off from God, which means that they will have cut themselves off from the source of all goodness. They will even lose the things that they thought were their own, like the ability to love and the ability to be thankful. They will be in a state of perpetual hatred, fear and distrust, completely unable to form any friendship or any co-operative relationship.

3. Satan does not love sin for its own sake. He hates God, he hates whatever comes from God, and he seeks to destroy and corrupt out of sheer spite. He is not some cheery hedonist, but a manipulative, destructive, narcissist. 

4. SIn, by its nature, is always a slippery slope to misery. Drunkenness and drug abuse destroys rationality, envy leads to quarrels and resentment, lies lead to distrust, pornography leads to sad and empty addiction, and so on. Sometimes these are very gradual descents, and sometimes people manage to break free of a sin before they suffer great harm, but it would be impossible to indulge eternally in any sin without corruption setting in.

The flames of hell are figurative, not literal.  Of course fire cannot hurt nor harm a disembodied spirit.  Hell is bad enough that it gets the point across, however.

It isn't there to torment us.  It's there to purify us of our sins if we refuse to repent, or murder, or sin against the Holy Ghost.  But hell is never forever.  Rev. 20:12-14 shows that death and hell must both be emptied and destroyed so that the resurrections and final judgment can take place.  There is only finite punishment for finite sin.

No, there will be no sin in hell.  People there concentrate on getting through the punishment for sin.  They wouldn't voluntarily add to the length of their sentences.

God created Hell so we don't need to be lost to him forever.  Satan has nothing to do with it.

Besides, how much debauchery do you think disembodied spirits can engage in?

Then you're not the brightest bulb in the box, the sharpest tool in the .  .  .  

Satan is tormented in the lake of fire.  He isn't the "CEO".  Satan will be tormented forever.  Fallen angels can't avoid eternal torment. Man can avoid eternal torment.   The Lord Jesus Christ is God, and He loves you.  Believing in the Lord Jesus Christ means being forgiven all sins past and future, and means going to heaven and not hell.  Death leads to immediate heaven or hell, and it is too late to be saved, after death.  All believers still sin.  See 1 John 1:8.  To be in heaven and not hell, believe in the Lord Jesus Christ, who is God and who died on the cross and shed His blood to pay for all of our sins in full, and who was buried, and who resurrected from the dead.  The only way to avoid hell is by believing in the Lord Jesus Christ, without adding any of your own works.  See Romans 4:5, 1 Corinthians 15:3-4, and John 3:16.  P.S. Eternal torment is true, as Revelation and many other places tell us.

You can imagine heaven and hell to be anything you want..

Revelations states you are destroyed in the fires not tormented, you should try actually reading the bible. Its somewhere around Revelations 18 to 20,  I narrowed it down to 2 chapters for you.