What is the reason for Allah's forgiveness of the sins of some of His servants and not others, and does His forgiveness include those who commit major sins and repent, among them the repentant murderer?

Indeed, Allah does not forgive that partners be associated with Him, but He forgives what is less than that to whomever He wills. Sins less than shirk (associating partners with Allah) are forgiven by Allah, and the one who commits them is under Allah's will. Major sins are divided into two categories: those that are solely between the servant and his Lord, such as adultery and drinking alcohol; for these, repentance with concealment suffices. And those that involve the rights of a human being, such as murder and theft; for these, repentance requires returning the rights to their owners or seeking their forgiveness. Otherwise, on the Day of Judgment, good deeds will be taken from the wrongdoer or bad deeds from the wronged person will be cast upon him. As for the right of the murdered person, there is a difference of opinion among scholars, but sincere repentance lifts the right of Allah. However, the rights of human beings remain, which must be fulfilled or their forgiveness sought. If one does that, then he has fulfilled his duty regarding their rights, and that is part of the completeness of repentance.