Q. We know that God will one day destroy Satan. but why didn’t God throw Satan into lake of fire immediately after he sinned?
A. When Satan sinned, God had a rebellion on His hands. It wasn’t a surprise to Him when it happened, but it did raise two questions that had to be answered. Was He righteous and just in condemning Satan (and by extension all who would follow him), and was His condemnation a sign that He really didn’t have a true sense of love and mercy for His creation?
To answer these questions God created man, a being similar to angels in that both have intellect, agency (free will) and eternal life. Then He gave man a set of laws to live by. If man kept God’s laws he would dwell with God forever, but if he broke even one he would be condemned forever. There would be no loopholes, no favoritism, no exceptions. Righteous laws, perfect justice.
Knowing that man could never keep His laws and would be condemned to eternal punishment, God became a man Himself and bore the penalty for all of man’s sins saying that whoever agreed to accept this substitution would be regarded as though he had never sinned and would have eternal life. Unconditional love, unquestioned mercy.
All of God’s righteous requirements were met by this, there’s no contradiction between His word and His deed. His justice, His righteousness, His mercy, and His love were displayed simultaneously, and because He left the decision to man to accept or reject His act of unconditional love, He did so without violating man’s free will.
At the End of the Age when God gathers all of His creation for judgment, both those who want Him out of their lives and those who want Him in their live will receive exactly what they asked for forever. It’s absolutely fair and the questions about God’s motives and His integrity will be resolved.