Let me try a very short answer which I am sure will only produce more questions, but anyway. There are other parts of this web page which talk about the cross extensively, please have a look at the Cross of Christ page. The Bible is full of strong statements about the holiness of God. God made it very clear, before the first sin, that the consequence of sin is death, and that is reiterated throughout the Bible.
Sin is rebellion against the highest authority.
It is an offense against God's holiness and right to institute all laws. It is basically saying that we know better what is right and wrong and we disregard God's authority. But God is not only holy and just, he is full of love and does not want that any should perish. God's love and mercy moved him to pay himself for it, since he doesn't want to destroy us although we would have deserved it. Jesus death on the cross is the substitutionary payment of all of sins death penalty of mankind.