How can we reconcile the scholars' saying that the perpetrator of a major sin is under Allah's will, with their saying that whoever's good deeds outweigh their bad deeds is among the people of Paradise?

The اهل السنة (people of the Sunnah) agree that a person who repents sincerely will not be punished for their major sin. As for one who dies without repenting, their fate is subject to Allah's will: if He wills, He may forgive them, and if He wills, He may punish them, except for shirk (associating partners with Allah), which He does not forgive. Whoever's good deeds outweigh their bad deeds will enter Paradise. Whoever's bad deeds outweigh their good deeds will enter Hellfire, and then their ultimate destination will be Paradise. There is no contradiction between a disobedient person being subject to Allah's will and someone whose bad deeds outweigh their good deeds entering Hellfire, for whoever Allah wills to forgive, He will subject them to an easy reckoning. As for those who enter Hellfire, they are those whose reckoning was thoroughly scrutinized.