The fairness of God falls under constant attack, even within the Church itself. Outside the Church, there are those who understand salvation so little that when they see someone as particularly undeserving of salvation, they take offense to them receiving it. I remember in particular, during the earliest days of my faith, one girl asked me "why should a murderer in prison go to Heaven while I go to Hell?"
It isn't especially hard to convince most people that if there is a Hell, certain people deserve to go there. Jeffrey Dahmer is an obvious example. While he is most likely in Heaven, he is quite well known for his cannibalistic, and even necrophilic behavior. If anyone deserves Hell, it's him. As for this sweet young woman, does she really deserve that?
The thing we miss, however, is that we all deserve Hell. God isn't looking at us and thinking "this murderer's actually pretty cool, he totally deserves Heaven. But this kind woman... ick, I don't like her. BUUUUUUURN!" No, what He's looking at is a bunch of sinners, all of whom, in some way, have earned His wrath. From the worst to the greatest, we've fallen short in some way.
The obvious flip side of this is that none of us deserve Heaven, either. A murderer does not deserve Heaven. You, reading this, do not deserve Heaven. I do not deserve Heaven. If God judges us all according to what we have done, not a single person will be good enough to merit His eternal blessing.
We actually understand this to a degree. Most people are quite happy to kill a bug that annoys us. Its crime? Living the way bugs live. Its punishment? Poison, sticky paper, drowning, or being crushed into a sticky, yet fully conscious mess.
The relationship between us and God is a lot closer than between us and a bug, and yet the difference between us is far greater. God is the origin of all things. The universe which holds the stars we sleep under, the sun which lights and warms our world, the ground we walk on, the food we eat, the air we breathe, the water we drink, even the very bodies we own, all of it comes from Him, and is sustained by Him. And we were created for a purpose, the fulfillment of which our lives have always been conditional upon.
Just as we are above a bug, God is far above us. He owes us nothing good. In fact, He Himself is perfect. There is not the slightest imperfection in Him. That means even though we are nothing significant compared to Him, we are still significant to Him. So much so that our evil deeds, though they do Him no personal harm, warrant His eternal wrath. That's just what happens when perfection meets imperfection. When light enters the dark, there is no more darkness, and God is the brightest light of all.
This makes it incredible that even one person goes to Heaven. In fact, when you consider the fact substitution is the only way for God to forgive sin and still remain just, it's positively fantastic God even bothers. Of course, it can be assumed the objects of wrath would still look upon that one lucky sinner with disgust and resentment, but it would still be fair to us if we were condemned in spite of him. Why? Simply because it would be fair if we were the only people in existence and still went to Hell.
See, when a criminal stands in a courtroom, the judge must only take him into account. Specifically, his crimes. His good deeds? Mere bribery. You could be Mother Theresa herself, one crime is enough to condemn you, because that is the only appropriate approach to crime.
But Jesus never committed any crimes. The sinful world might disagree, but morality isn't relative; God is judge. Thus, since Jesus met God's standards, He is, by definition, the perfect man. Yet He still received the wrath of God. Jesus died for sins He didn't commit, because He was saving the people who committed them. That is, Jesus died for the sins of the world.
Because of that, actually, everyone can go to Heaven. That's why it's fair when murderers go, but stand up citizens don't. Those who go to Hell go there because they deserve it. Those who go to Heaven instead of Hell do not deserve Heaven, they deserve Hell, but Jesus got what they deserve, and so they get what He deserves. Out of love, Jesus bought eternal life for everyone who believes. The murderer goes to Heaven because he believes. The unbeliever goes to Hell because even though they could have gone to Heaven, they chose not to. They rejected the gift. They chose to receive what they deserve. You literally cannot get more fair than that; every sin is dealt with. Every debt is paid in the end. It's just a question of whether you pay your own debt, or, through a relationship with Christ, you receive a reward He bought you.