Why didn’t God simply destroy Satan from the beginning?
God did not tell us so this requires me speculating, so please take my answer for what it is worth/with a grain of salt.
Why did God not destroy you when you sinned? Because he loves you and because he wants you to repent and have a relationship with him.
God created Satan, but Satan was not evil at first. Surely God would not have destroyed Satan at the beginning, as he was not evil. He rebelled against God and became evil. I have to assume that Satan and other angels must have "free will." Therefore angels (and Satan is a fallen angel) have the possibility to rebel against God and to become a force for evil not good. According to the Bible, God will judge Satan at the end of time–on Judgment Day (see Revelation 20). In the end, God will have to explain why he chooses to let evil people (like us) or evil angels (like Satan) continue to exist, but he does. I believe he does so because he loves us and he wants us to repent and return to him.
John Oakes PhD