Why is the devil mankind's adversary?
Lucifer (Satan) was one of God's most magnificent angels. Lucifer became filled with selfish pride and rebelled against his Creator, God Almighty. He thought he could outdo God and he set forth to overrule Him. 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 is at the heart of the first rebellion. The mysterious infliction called "sin" is the supernatural infection that entered the Garden of Eden with Satan (the serpent) 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 (study Genesis 3 about the temptation and fall of mankind).
God renamed Lucifer, Satan, "the serpent" and "the devil." Satan 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 sins of all mankind.
Man must accept this free gift of God's grace but He does not force it on him, whom he made in His own image. Perhaps much of Lucifer's hatred for mankind is in part because he was made in the image of God, and is destined for the highest portals of heaven where he will be given reign and rule over God's creations, including angels; whom the Bible says the redeemed will judge.
"Know ye not that we shall judge angels? How much more things that pertain to this life?" (1 Corinthians 6:3).