"Satan" literally means a "stumbling block". Christianity has always recognized the existence of the fallen angels, Satan being the lead. These purely spiritual beings, created good by God but confirmed in evil by their own free choosing, can and do in fact prowl about the world trying to tempt souls away from God. It is our duty, with the help of the Grace of God, to resist them and turn to God. Failure to do so on our part essentially puts us "in league" with Satan.
Satan is the tempter (and a powerful one at that). If we fall into his trap, we retain full responsibility for doing so. I fail to see how this is antithetical to monotheism.
"If I knew God, I'd be Him." Though I doubt that man needs any agents to lead him into evil, which God creates, as One.