Why is the devil mankind's adversary?
Lucifer, one of God's most magnificent angels, was created perfectly. For some reason we human beings have not been told, Lucifer became filled with self-pride, and rebelled against his Creator, God. He led one-third of the angelic hosts in a revolt that has been raging ever since. (Read Isaiah 14:12-14.)
Lucifer's desire to be equal to or even higher than God lies at the heart of the rebellion. The mysterious thing called "sin" is the supernatural infection that entered the Garden of Eden with Lucifer when he tempted Eve, then her husband Adam, to disobey God by eating of the forbidden fruit of the tree of the knowledge of good and evil.
Lucifer, renamed "the serpent" and "the devil" by God, apparently wanted --and still wants-- mankind to worship him, not God. The Devil's all-consuming desire is to keep people from coming back to God through redemption found only in God's Son, Jesus Christ, who died on the cross as a sacrifice to cover the sin of all mankind. Man must accept this grace gift, however; God doesn't force it on man, whom he made in His own image. Perhaps therein lies much of Lucifer's hatred for mankind. Man, made in the image of God, is destined for the highest portals of Heaven, where he will be given reign and rule over God's created things, including angels, whom man, the Bible says, will judge.
"Know ye not that we shall judge angels? how much more things that pertain to this life?" (1 Cor. 6:3).