I disagree. Christianity does not consider Satan a necessary "soteriological" factor, let alone part of the divine Economy of our salvation! Nor do we consider Satan to be God's obedient servant! In Chrisitanity, Satan is a fallen angel who rebelled (and rebells) against God.
Satan is as completely subject to the Will of God as any other creature. Period
Being subject to the will of God doesn't mean God is willing to stop evil.