The second brother lives in disobedience after maturity, and the dialogue assigns punishment to him. The case arises in the context of Muʿtazilī debates concerning grave sin, faith, and final recompense.
It must not be converted into a categorical claim that any particular sinful Muslim is certainly destined for Hell. A hypothetical premise differs from verified knowledge of an individual's final state. The scenario asks why, if a person's later disobedience were known, an earlier death would not be required as the supposedly “best” alternative for that person.
The logical target is the alleged universal obligation of aṣlaḥ, not a judgment upon an actual individual.