Why can't God forgive a sinner without having to kill Jesus the righteous one?

Non-Christians raise many objections to the atoning death of Jesus Christ, who is the Lamb of God. One of the most important objections is that God being the Almighty and Sovereign should be able to do whatever that pleases him to do, except illogical ones. It is God's prerogative either to forgive or not forgive anyone. Therefore, if God the Almighty wants to forgive a sinner He should be able to do that without having to make a payment or a substitution on behalf of the sinner. If the God of the Bible cannot forgive sinners this way then how can He be the Almighty God? My above question is mainly to the protestant Christians.