God is Just

(Act 10:34) Then Peter opened his mouth, and said, Of a truth I perceive that God is no respecter of persons:

 "It is really unfair" is a common statement for one who has been judged unjustly. Every time someone in authority takes a position that seems unfair to others, the ones to whom the ruling seems unfair feel bad. The question usually comes, why do people feel that fairness was not exercised? It is because they were created in the image of God, and they know what is morally right or wrong inherently within them. Any ruling not in line with that rubs them the wrong way.

God's justice demands punishment for wrongdoing. The question is, "Is God punishing me for my sin that I do in the present day-to-day?" If He is doing so, then He Himself is not fair towards His Son, whom He punished on behalf of the sin of the entire world. When Adam and Eve sinned, God throughout the ages instituted animal sacrifices to atone for man's sin. An animal to die instead of man, otherwise if that was not so, then it would not be fair to the universal courts of justice that sin went unpunished. The death of an animal appeased God's wrath; otherwise, man had to die instead. All these animal sacrifices were shadows of the ultimate sacrifice, which was Jesus, that was yet to come.

God's justice has reserved punishment for sin eternally for all who would not accept the forgiveness of their sin by simply believing in Jesus Christ in the lake of fire. That is just at least. In fact, they won't be punished for sins they commit, but for not believing in Jesus to receive righteousness as a gift. 

Further scriptures

(Joh 16:8)  And when he is come, he will reprove the world of sin, and of righteousness, and of judgment: (Joh 16:9)  Of sin, because they believe not on me; (Joh 16:10)  Of righteousness, because I go to my Father, and ye see me no more; (Joh 16:11)  Of judgment, because the prince of this world is judged.