In Surah e Shams Ayah 8 Allah explained that He has put in your instinct the basic understanding of good and bad things. In other place Allah said, He also put in our instinct that Allah is one.Despite of all these, he then sent down Messengers for inform the people about Allah and His teaching.And revealed books and Sahifaas on them and showed “Mujzas” to them that they accept Islam but they refused to accept Islam why because of the fact that their ancestors were following this religion for which they have no evidence and for status and money.However, Allah has promised those who accept and follow message of Allah to the Heaven and blessings which are unimaginable and they will always remain alive in Heaven which is far beyond great blessings of accepting Islam and in Surah Teen last verse, he said, “Is Allah is not the biggest Judge among all.”So when you think that justice is necessary if someone does some wrong doings then he should be punished and Allah has made many efforts to save people from this severe punishment then why hell is unjust. Disbelievers not only refuse to accept Islam after they were fully aware that Prophet Mohammad SAW is the true Prophet of Allah and Quran is the book of Allah but they refuse.Allah has said in Surah Al Bakarah verse 6 explains the punishment of such refusal.Allah after given them plenty of time announced such strict verdict.
Hope you will understand it.