The concept of predestination is highly discussed, and in the end, the best thing to come to is accept that it is not possible for us to understand fully.
We believe two things: 1) Everything is known by Allah and predestined, and 2) people choose whatever actions they do and that leads to their fate in Hell or Heaven.
The difference between the two is that Allah does not force people to do anything. He simply knows what they will choose and predestined it.
On the other hand, people aren't forced to do things Allah predestined. Rather, they do what Allah predestined because they chose it, not because they were forced.
Allah is also the Most Just and Most Merciful. He doesn't punish people for something they were forced to do. This is apparent in our law: a person isn't punished for eating pork or drinking alcohol when he is starving.
So, it is clear that predestination doesn't force people to do anything. Rather, it is simply the fact that Allah had knowledge of the future beforehand, wrote it in a book, and created the future. He did not force people.
If it is too difficult for you to understand, then don't try to delve deep into it. This is a highly discussed topic and there are many books written on the concept qadr. Qadr (predestination) is the sixth pillar of faith.
Believe that Allah predestined things and people weren't forced to do them (rather they chose in free will), and you will be fine. Do not delve deeper into it if you cannot understand it.
Some people ask: How can it be a free choice when I can only do one thing?
Answer: The reason you can do one thing is because Allah knew what you would choose! It is not because Allah limited your choices in such a way that you were unable to choose what you actually wanted.
A hadith is narrated about this topic that:
The Prophet (SAW) said: "There is not one amongst you whom a seat in
Paradise or Hell has not been allotted and about whom it has not been
written down whether he would be an evil person or a blessed person."
Someone asked: "Allah's Messenger, should we not then depend upon our
destiny and abandon our deeds? "
Then, the Prophet said: "Acts of everyone will be facilitated in that
which has been created for him so that whoever belongs to the company
of the blessed will have good works made easier for him and whoever
belongs to the unfortunate ones will have evil acts made easier for
The meaning of this hadith is that even when everything is predestined, we should still do acts because we do not know the destiny. You do not know whether you will enter Hell or Heaven. That makes all the difference. The actions you do now will decide whether you were destined for Hell or Heaven, even if from Allah's pov, it is already decided. From our pov, our actions will decide it.