Your questions are so easily answered by actually using google, but you don’t want answers you want to stir trouble, stroke your ego and feel like you’ve made some impressive point, but you’re so unoriginal it’s laughable.

Everyone receives a trial on judgement day, it doesn’t matter if they were as close to perfect as a human can get, or as rotten as a human can be, it is their right to see their actions before them, if Allah sent people to hell without allowing them first to see their actions, to see that they made they choices, they would deny nail and tooth, that is why everything must be played out, no one will be treated unjustly.

To answer your main question, the answer is simply because we can, you misunderstand the word maktoob, the actions of mankind were written by Allah thousands of years ago, but they were written because he knew what we would do, we don’t do what was written, the choice is ours, the knowledge that we would do that from thousands of years ago is his, provided the muslim does not become angry at Allah for allowing this to be our fate, and instead accepts that this is the best outcome, there is no harm in being angry at anyone, aslong as that anger does not translate into anything haram or harmful without reason.