Free will was not initially installed for humans. It was for angels only. Angels' free will is advanced whilst for humans is basic( for lack of better words) Therefore, Angels nature is perfection. Humans and error are inseparable. And for this reason, an angel making a mistake can never ask for God's forgiveness – for it's not merely a mistake. On the other hand, human beings can be pardoned by God endless times if they make a mistake. Everyone is a sinner, however, those who will enter heaven are those who put themselves in a position of spotting mistakes, admit them and in turn correct them. A fallen angel is basically one that decided to sacrifice itself to meddle with God's purpose.