Hell Doesn't Exist, Right?

Hell absolutely exists.  Jesus spoke often of its existence and warned sinners of the consequences of their obstinate refusal to reject evil.  
 

Yeah, But the Devil Is Just a Concept, Not a Being?

The Devil, aka "Lucifer" or "Satan", is a real, created angelic being who rebelled against God and brought with him to hell countless other fallen angels.  It is not possible that God would tempt man to sin.  If sin exists, the Temptor exists.  The Temptor is the Devil.  
 

I Heard Hell Is Empty

Unfortunately, that doesn't seem to be the case.  First, our Lord was emphatic that the road to heaven is "narrow" and "few find it".  Second, after His resurrection, the Lord specifically mentioned that the deceased Judas "went to his own place".  Third, we know that the Lord only promised one of the thieves He was crucified between that he would go to heaven.  Fourth, the Lord promised there would be goats and sheep to be separated at the Final Judgment.  Although we pray that all souls go to Heaven, there is a preponderence of evidence in Scripture, Tradition, and the Magisterium of the Church that not everyone accepts Christ's gift of mercy in the end, and are therefore damned for all eternity.  
 

How Does Someone End Up in Hell?

Hell is a necessary punishment caused by a lack of sanctifying grace in the soul at the moment of death.  Without sanctifying grace, we cannot be saved.  
 

How Does Someone Lose Sanctifying Grace?

Mortal sin severs the relationship between God and our soul by extinguishing sanctifying grace.  If mortal sin remains unrepented, it becomes an eternal impediment upon death.  Every time we go to Confession, however, we are freed of mortal sin, sanctifying grace is restored, and our relationship with God picks up where it left off.  The importance of having a consistent ethic of going to Confession and receiving absolution cannot be overstated.
 

Can the Worst Human Being Imaginable Avoid Hell if They Repent Right Before Dying? 

Yes.  The worst person in the world is salvagable until his/her last breath.  The Church does not proclaim anyone to be in Hell like they proclaim saints to be in heaven.  Scripture and tradition argue strongly for Judas to be among the damned (see above) but we do not know the final moment of life for anyone else.  Common sense may tell us certain profoundly evil individuals of public notoriety could not have had time to repent, given their particular demise, but we err on the side of charity that, even in the worst of the worst, God sees a soul that can be saved until their final moment. 
 

First, we don't know who is and who isn't in hell.  Culpability is dependent on factors you and I cannot know, which God does know - i.e., the secrets of the heart.  

Second, we know from faith that God exists outside of time.  Thus, the prayers you say today can be applied to the soul who perished years, decades, even centuries ago.  In other words, at the moment this ostensibly wicked individual is perishing, God has already seen the prayers you've said for that individual today.  Just like the friends of the crippled man brought to Jesus, your act of faith could win a miracle of grace of final repentance to any soul at any time.  We don't consign anyone into hell, because every soul is capable of being saved. We must be constantly aware that the very grace we receive is a gift, and oftentimes a gift that comes from another person's intervention on our behalf -- maybe even a descendent yet to be born, generations to come.   But that's the depth of God's mercy. 

What a great consolation to know that if we have a departed loved one whose fate we are concerned over, we can still be part of their hopeful salvation.  It just might be God's will that you will bring to heaven souls that had no one else to pray for them in their time of greatest need.