No, it’s their failure to believe in God, which is their own sin.
That just isn’t true. The expectation of justice is both temporal and eternal. The desire to see temporal justice meted out is a real thing that can affect a person’s ability to forgive. If it wasn’t important God would not have needed to set up governments and any penalties for anything bad here because “well he will just take care of it later in eternity”.
Yet if the murderer roams free to continue to live life happy without consequence, and continue to murder at will, how can the living victims that survive, or are relatives of the murdered, forgive?
You grossly oversimplfiy and downplay the role that temporal justice plays in the victim’s ability to forgive. God doesn’t downplay it, I don’t kow why you do.