Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
Allah, the Exalted, created creation for a great wisdom, and not merely so that some of them would enter Paradise and others would enter Hell. Allah says:
﴿And I did not create jinn and mankind except to worship Me.﴾ [Adh-Dhariyat: 56]
The purpose of creation is for people to know their Lord, affirm His oneness, worship Him, and obey Him.
As for entering Paradise or Hell, this is a recompense resulting from the deeds that people perform in this worldly life, even though Allah's prior knowledge encompassed what they would choose and do. His prior knowledge does not mean that He forced them to perform their actions. Rather, He created them, gave them the ability and choice, sent messengers to them, revealed the Scriptures, and established the proof against them. Allah says:
﴿Messengers as bringers of good tidings and warners, so that mankind would have no argument against Allah after the messengers.﴾ [An-Nisa: 165]
Allah does not punish anyone merely because of what He knew about them beforehand. Rather, He recompenses them for what they did and chose after the proof had been established against them. This is why the people of Hell will say:
﴿Did there not come to you a warner? They will say, 'Yes indeed.'﴾ [Al-Mulk: 8-9]
As for Iblis, he was not sent down to earth because he was deprived of repentance without cause. Rather, this occurred after he disobeyed Allah's command, became arrogant, refused to repent, and did not feel remorse. Instead, he persisted in disbelief and defiance and asked Allah to grant him respite until the appointed time.
His remaining until the Day of Resurrection was therefore for a wisdom, including testing the servants, making evident the obedience of those who obey and the disobedience of those who disobey, and distinguishing the people of faith from the people of misguidance.
The difference between Adam and Iblis is that Adam, peace be upon him, committed a sin and then repented, so Allah accepted his repentance and guided him. As for Iblis, he persisted in his sin, became arrogant, and did not repent.
In conclusion: Allah's prior knowledge of the destiny of His servants does not mean that He created them merely to punish some of them, nor does it mean that He forced them into disbelief or disobedience. Rather, He created them to worship Him, sent messengers to them, enabled them to act and choose, and then recompenses them for their deeds after establishing the proof against them. All of this falls within Allah's knowledge, wisdom, and justice.
And Allah, the Exalted, knows best.