"Why did God have to kill His Son?" Answer: Because God is just, and because God is love.

For Him to continuously overlook disobedience would be to indicate that it really doesn't matter to Him how people behave toward Him or toward one another, and that would be unjust. But for him to mete out to us what we fully deserve would leave us destined for destruction, because we cannot do anything to justify ourselves. And so,

God put forward [Jesus] as a propitiation by his blood, to be received by faith ... so that he might be just and the justifier of the one who has faith in Jesus. -- Romans 3:25-26 ESV

Humankind was made to be fruitful (Gen. 1:28), to God's glory (John 15:8) and our joy (John 15:11). The atoning sacrifice of Jesus means that the potential for fruitfulness and joy has not been obliterated by human sin but is sustained by our justification and all the other gifts that God continues to give (Romans 12:6; 2 Peter 1:3-8).