We cannot assume that angels are immortal and share the same kind of spirit God Almighty has; we cannot assume they are indestructible.
The demons who already inhabit the earth look upon us as interlopers. We need to monitor our thought impulses, lest we be bothered by demons.
The origins of our adversary, Satan the Devil, and his host of fallen angels or demons. God has promised us protection if we yield to and obey Him.