God, in his simplicity, has various perfections.
We know that God possesses various perfections, goodness, wisdom, love, etc.; and not only in the sense that He can create them, but that He Himself, really has them (they really exist in Him), and, they are in Him in the most eminent way (God is infinite). God does not only possess goodness, wisdom, love, etc., but He is Goodness, Wisdom, Love, etc.
However, God is simple (not in the sense that it is easy to understand Him, but in the sense that He is not composed of parts; there is no distinction between His essence and existence). Yet, these various and really distinct perfections (in created order) are not synonymous in God (hence we can not say that God pardons by His justice). In God, these perfections really converge in the most eminent way to one all-encompassing idea of Deity which is God Himself.
Creation is a sharing of the treasure of perfections found in God.
God manifests His infinite plentitude of perfections, His Glory, trough an act of creation. Since no created being can be absolutely simple and capture all that God has in his infinite perfection, God creates many beings to manifest His Glory (of course, no matter how many created things there are, they can not exhaust God's Glory).
Christ's death is concentrated manifestation of His infinite Goodness and mercy toward human beings.
Christ came to the world to save us, He suffered for our sake. Out of his goodness and mercy, He came to redeem us by His salvific love. God offered salvation to mankind (and this is His completely freely given gift to us, which He did not have to offer to us) out of his sheer mercy. There is nothing in us that "deserves" to be loved, we are from ourselves nothing, but by God's mercy, we are able to see His very essence. Why did God choose to manifest his Goodness in this extraordinarily high measure? Because he freely chose so, it is his simple will (which is sufficient reason to itself).