So saying that he HAD to kill jesus to remove sin is stupid. He dosn't HAVE to do anything. He's omnipotent he can do anything he wants.
You are 100% correct. God is not limited in any way, shape, or form. It is man's tendency to limit God because of our limited and closed minds.
Allow me to throw in my two cents.
Although God does not
have to do anything, the fact that he
did do something is significant in the sense that it must have had a purpose (not many would argue that God is an arbitrary god). The motivation in which He chose to do this "something" may or may not be evident in the Bible or any religious text. On the same premise that God does not have to do anything, God does not
have to reveal His purpose.
From a mortal's perspective attempting to probe the mind of God, my answer to why God would choose to redeem mankind through the blood of Christ might be because of God's desire to express His love in the ultimate way; that is the death of His Son. Such a sacrifice evokes a stronger desire to seek and worship God because of it's implications. Who would not be eternally grateful that another's life was given in place of your own? Most Sunday worship music revolves around the cross and how God chose to express His love. I am sure that God knew this response to His system of atonement when He decided to carry out His will in such a way.
Paul also preaches that if it were not for the resurrection of Christ, then a Christian's faith would be in vain. I am sure that the resurrection also played a key role in God's purpose for the death of Jesus. Despite the torture and humiliation that Christ had to endure, His glorification through the resurrection was a showcase of the power of God and fulfillment of prophesy.
Perhaps the death and resurrection was God's way of expressing His love and power, respectively. God was not subject to doing it, but it was the course of action he predetermined for humanity.