How does Islam justify non-guided people going to hell and what is the point of free will if the future is already written?

We are all Muslims because we are either raised up as Muslims or at some point in life reverted to Islam. But what about others, the ones who are not being guided is it just their fault that they have not become Muslims. They would have done what the Islam teaches them to do if they had been guided and it is said that those who have not accepted Islam will go to jahannam. So how is this justified? Also, we know that the future is already written and if it's already written what's the point of free will?