The Bible says that God created hell for Satan and the wicked angels who rebelled against Him, but there are people in hell also (Matthew 25:41). Both angelic beings and human beings are in hell for the same reason, sin (Romans 6:23).
Because God is completely righteous and morally perfect (Psalm 18:30), He always does what is right—there is no “darkness” in God, not the smallest speck of imperfection (1 John 1:5). God Himself is the standard for what is right, good, and moral. If it were not for God being the standard of moral perfection, created beings would have nothing to measure themselves against. In other words, if God is perfectly righteous, then anything that falls short of said perfection is sinful, and every human being who has ever lived, since Adam’s fall from grace, has committed sin (Romans 3:23). Because Adam sinned, the entire human race now has a sinful nature (Romans 5:12). But people do not go to hell because of Adam’s sin; they go to hell because of their own sin, which they freely choose (James 1:13–16).
Since God is eternal, immutable, and infinite, and all sins are fundamentally against God, God has decreed the just punishment for sin must also be eternal (Matthew 25:46). There is another aspect to consider, which is that God also created people to live eternally. So when someone commits a sin against another person, the offended person has also been eternally wronged.
God, therefore, has deemed all who commit sin will go to hell because they have failed to meet His righteous standard; they have broken His Law of moral perfection. If God did not send people to hell for breaking His laws, it could be said that God is not just (Psalm 7:11). A good analogy is a court of law with a judge and a lawbreaker. A just judge will always convict the person who has been found guilty. If that judge did not pursue justice for the crime, he would not be a just judge (Deuteronomy 32:4).
However, the good news is that God is also merciful. In His rich mercy, He made a way for sinners to avoid the punishment of hell by trusting in the atoning work of His Son, Jesus Christ (Mark 16:16). For Christians, the penalty of sin has been removed and placed upon Christ on the cross (1 Peter 2:24). Because of the sacrifice of Christ, God is still just—the sin is punished—yet He is also merciful to all who believe.