what does the bible say?
Not much is said about why God allowed Satan and the demons to sin, but from what we do know, God found it best to happen that way. God created Satan and the other angels with the ability to make decisions—to reject or respond to Him. Unfortunately, Satan desired what he did not have and wanted to be God. Other angels followed in his footsteps. Just as humanity is given the opportunity to reject or respond to Christ, so angels did as well. Although Satan and the demons sinned against God and continue to try and bring destruction today, their end is sure: the lake of fire. However, Christ came and died for our sins and brought salvation to humanity. God offers us salvation and to turn from our sin until our last breath; whereas, the angels were not given the opportunity for salvation.