If the serpent or snake is a symbol of Satan who tempted Adam and Eve in the Garden of Eden and then was cursed by God; why does Christ also use the symbol of the fiery serpent to represent himself? All symbols and all creation are Gods. Satan is the one who impersonated, counterfeits, steals and corrupts a symbol. Nothing belongs to Satan as he can not create anything.
Why is a venemous serpent used as a great symbol of Christ? Christ as a healing serpent is a powerful and suprisingly accurate symbol. Think about how you treat a venomous snake bite-- with antivenom from another venemous snake. Antiserum is created by harvesting and injecting the snake venom into a horse or rabbit forming antibodies against the venom. Then you administer the healing blood or serum containing these antibodies into the envenomated patient.
So, just think about this miraculous metaphor of Christ's Atonement. Like anti-venom, only after taking upon Himself the poison of our sins and death does Christ's blood become the needed healing anti-serum. All those who look to Christ and take upon themselves His blood will be healed and live eternally.
Like antivenom, convelescent serum is the only effective treatment for the most deadly hemorrhagic viruses like Ebola. The antigen/antibody principle of immunity is also the basis for immunization and vaccination technology.