Why did God have His Son killed instead of just killing the devil and ending everything?

Answer:

Ending Satan immediately would seem like a just response to the misery Satan has caused in this world. However, it wouldn't solve the problem. Satan corrupted the world by introducing sin. The effects of sin would still remain, even after Satan was gone. Who would rescue mankind?

"For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life" (John 3:16).

This verse is so familiar that we sometimes forget the depth of what is said. God loved His creation so much that He was willing for the Son of God to die in order to have a just way of rescuing them from their sins.

Meanwhile, Satan was left to roam to cause a distinction between those of mankind who desired God and those who did not care.