Does thought exist before you have it? Where is the seat of thought? Is it in our brain or does it come about from unseen influences, such as angels and demons? Do we truly have thoughts independently or are they given to us? Sin is defined in the bible as "4 Everyone who sins breaks the law; in fact, sin is lawlessness. 5 But you know that he appeared so that he might take away our sins. And in him is no sin. " (1 Pet 3:4-5) Sin is Lawlessness. So sin itself may not have ever occurred before Satan, but was a possibility. Also consider this. Apart from "law" there is no sin. Good and evil can only be defined if "law" exists. (See Rom 4:13-15) "13 It was not through the law that Abraham and his offspring received the promise that he would be heir of the world, but through the righteousness that comes by faith. 14 For if those who depend on the law are heirs, faith means nothing and the promise is worthless, 15 because the law brings wrath. And where there is no law there is no transgression." Where there is no law there is no transgression. Now think about a pre-existence, call it Heaven, where "purpose" exists, but there is no "law" defining how things should "be". No sin exists there, no matter what the "purpose" may be. And even if something exists which has "no purpose", there is still no sin, because there is no law. And that is probably what "Heaven" was like before the fall of Satan. Also consider Eph 6, that there are "rulers" , "powers", "spiritual forces of wickedness in heavenly places"; which are NOT flesh and blood (v12) that we are at war with. These "rulers" may not be in league with Satan and not under his command, but are independently wicked. Some of them are said to be "in heavenly places"; but remember they are not flesh and blood (aliens, EBE's, humans, etc). "In heavenly places" refers to regions of heaven where God resides that wickedness still has a foothold. Consider also that some demons were "bound" (See Jude), whereas some are not; ie. the devil and his angels, and these others mentioned in Ephesians.
My point is that Satan and his angels are not the only wicked ones we are at battle with.