Satan cannot be considered the King of Hell, as there is no such title. Hell is not a kingdom, but a place created in a separate space. Or to use a scientific term, it is a dimension from Heaven that will exist eternally to be the punishment itself. It was not initially created for people, but for the fallen angels. “Then [Jesus on His throne] will say to those on his left, ‘Depart from me, you cursed, into the eternal fire prepared for the devil and his angels’” (Matthew 25:41). If hell was made for the angels, and Satan in a fallen angel, he will not reign there. It will be the source of his torment.
Satan will not have power forever. In fact, his power is neutered even now. The only thing he can do is try to prevent people from accepting Jesus Christ as their Lord and Savior, vengefully trying to bring as many humans with him as possible. Praise God Jesus has made a path for anyone to repent and spend eternity in Heaven. Until Satan is forever cast into hell, “Be sober-minded; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour. Resist him, firm in your faith, knowing that the same kinds of suffering are being experienced by your brotherhood throughout the world. And after you have suffered a little while, the God of all grace, who has called you to his eternal glory in Christ, will himself restore, confirm, strengthen, and establish you” (1 Peter 5:8-10).