If Satan had not been bound by Christ's death and resurrection, then he would still have power over death, and no man could enter into eternal life. To be bound and cast in the pit, means that Satan is the one who is now bound by death [bottomless abyss]. It isn't a matter of Satan defeating God, it was simply that before Christ, Satan had power to keep God's people from entering into eternal life. Christ must be born a man, suffer and die and be raised again if He is to set the captives free. All those saints under the Old Covenant, who died in faith did not receive the promise of eternal inheritance. How could they since Christ had not yet come to free them from the power of death the devil held them in? Since the cross, not only have they gone spiritually to heaven, but we too go to heaven to be with the Lord at the moment of physical death.
Heb 2:14 Forasmuch then as the children are partakers of flesh and blood, he also himself likewise took part of the same; that through death he might destroy him that had the power of death, that is, the devil;
Heb 2:15 And deliver them who through fear of death were all their lifetime subject to bondage.
Heb 9:15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
Heb 9:16 For where a testament is, there must also of necessity be the death of the testator.
Heb 9:17 For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth.
Joh 12:24 Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.
Ro 14:9 For to this end Christ both died, and rose, and revived, that he might be Lord both of the dead and living.
Col 2:15 And having spoiled principalities and powers, he made a shew of them openly, triumphing over them in it.
Re 1:18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
Ho 13:14 I will ransom them from the power of the grave; I will redeem them from death: O death, I will be thy plagues; O grave, I will be thy destruction: repentance shall be hid from mine eyes.
1Jo 3:8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
So now Satan is the one who is bound in death, and it is Christ Who holds the keys of hell and death. Satan could never defeat God because Christ is the Lamb slain from before the foundation of the world. Eternal redemption in His blood was written in heaven for all who would be saved before the creation. Since Christ was victorious over death and the grave, Satan is left without strength, because even in a martyrs death he cannot keep God's elect from entering into His eternal Kingdom. That's why Christ tells us, "I will build my church; and the gates of hell shall not prevail against it." So you see Satan had to be bound by Christ so that all glory, honor and praise would be unto God alone!