What is the reason for Allah's forgiveness of the sins of some of His servants and not others, and does this forgiveness include those who committed major sins and repented, such as the repenting murderer?

Allah, by His grace, forgives all sins, save for polytheism (shirk), for whomever He wills. Major sins are of two types: those that are solely between a servant and his Lord, which are expiated by repentance, and concealment of which is commendable. The other type is that which involves a right of a human being, such as murder or theft. This type requires seeking absolution from the wronged party by restoring rights, or through retribution (qisas), or by forgiveness. Otherwise, the wronged party will take from the oppressor's good deeds on the Day of Judgment, or the wronged party's sins will be cast upon him. Sheikh al-Islam Ibn Taymiyyah held that one who kills unjustly incurs two rights: a right of Allah, which is forgiven by sincere repentance, and a right of human beings, from which the killer gains absolution through retribution, reconciliation, or forgiveness. Scholars have differed regarding the persistence of the murdered person's right over the repenting killer on the Day of Judgment.