There are people who complain that a God of love could never have devised an eternal hell. What is hell, and for whom was it designed? Hell is a place of torment and was initially designed for the devil and his angels, as a place of punishment for their rebellion against God's rule (Matthew 25:41). Remember that they saw Him and did His bidding (Hebrews 1:7). The devil must have convinced them that they could usurp God's throne and be the rulers.
There is a principle which we find in the Bible: people are divided into two groups, those who have God as their spiritual father and those who have the devil as their spiritual father (John 8:42-44). If a person has God as his father he wants to carry out His Father's desire. If the devil is his father he wants to carry out his desire. God is gathering a people for Himself and He will provide the renewed heaven and earth as an eternal home for them. Since the devil is also gathering a people for himself, they will share his eternal dwelling, which is hell.
Another principle is that God has limited Himself. He will not override a person's will. People are able to make their own autonomous decisions. They cannot claim that the devil made them do this or that. Man takes full responsibility for his decisions and lifestyle. That is why it is written: 'For we will all stand before God's judgment seat.... So then, each of us will give an account of himself to God' (Romans 14:12). If people were robots that could be influenced by a programmer to make wrong decisions, they could not be held responsible for their actions. The Bible illustrates that that is not true. People in the Biblical narrative gained or lost advantages as a result of their (free will) decisions. God held Adam and Eve responsible for their disobedience to Him and they lost their perfect home, provisions and wonderful life of peace and happiness (in the Garden of Eden); Cain was banished from his people, by God, for murdering his brother; the people in the time of Noah's flood died a watery death because of their wickedness and violence; et cetera. So there is not the slightest reason why God will absolve us from the responsibility and results of our decisions.
This means that God is fully aware of people's ability to comprehend the message of the Bible and make an informed decision about their eternal destination (this applies to those who live in countries where Bibles are available. One result of the world wide spread of the message of evolution is that people all over the globe have become much more aware of the fight against the Creator and supernatural creation as presented in the Bible, since natural, random evolution is intimately opposed to creation).
Now to answer whether God is unfair that He designed hell for the devil and his angels and that people can join the devil there: some nations might not understand it, but in South Africa it is very popular to live in what is called security villages. Walls, burglar proofing, alarm systems, watch dogs and electric fences are normal and almost a prerequisite. Why do people do that? It is to keep out criminals like thieves, murderers and rapists. Besides that, most countries have judicial systems with policemen, prisons, courts and judges. Why is that? It is because there are criminals that harm, rob and victimise law abiding citizens. They must be removed from society to protect the others.
God is planning a massive security complex. It will be the renewed heaven and earth and it will be a place of peace, happiness and safety. God will not allow criminals who will harm His people, there (if we desire peace, happiness and security on earth, why must the people of God be denied it eternally?). Hell is designed to keep criminals out of God's eternal society. On the renewed earth, God will be with His people (Revelation 21:3). 'Criminals' in that context are people who refused to love Jesus and abide by God's standards of doing unto others as you want them to do to you. They will be sent to hell to join the super rebel, the devil. That is the result of their own decision, since God has no favourites. Because God's new earth with the people on it is an eternal arrangement, hell will be eternal.
People who are adamant that God is unfair to exclude people who will harm others from His kingdom, do not go and live in dangerous countries without making provision for protection against criminals. So their accusation against God is not valid. Such people usually make sure that they live very safe and secure lives. They avoid places where they fear for their lives because you never know when you will be killed for your cell phone or a few paltry Rand (South African money). In addition many countries have rehabilitation centres where people can be helped if they have problems (psychologists and various others). God also has a rehabilitation centre that is run by the Holy Spirit of Jesus. Nobody is turned away. It is called the Church. The problem is that people want to break God's laws and be independent. Then they whine when it comes to accounting time. It is like people who don't want to invest in fitness and health, and then whine when others reap the fruit of their sweat, while they are sickly and unfit. People who think God is unfair, should talk to others whose relatives and friends have been killed, raped or robbed. Do they have such sympathy for the perpetrators that they think it is unfair to sentence them to jail?
We imitate God: because He is the Judge, we have judges; because He has a constitution (the laws of the kingdom), we have laws; because God has an eternal hell where criminals who refused rehabilitation will be confined, we have jails. We think jails are perfectly normal, because God thinks so too. Thus the law of God is very clear: no impure thing or person will ever enter the city of God (Revelation 21:27). Only those whose names have been written in the book of life, those who signed up to become citizens of God's Kingdom will be there.