I just wanted to ask why Shirk was the greatest sin in Islam and not something else like murder or rape? Because I was under the impression that if you hurt someone else then Allah won’t forgive you until that person does, and once that person does forgive you then it becomes between you and Allah. So why isn’t hurting someone else in cruel ways such as murder or rape one of the greatest sins instead of shirk?

All the sins can be forgiven by Allah because that person may still believe in Allah but committed a crime against humanity. Whereas one who doesn’t believe in Allahs oneness is a disbeliever