“Mankind as a whole”, with the exception of those souls who have committed themselves to Jesus Christ, i.e. the Blessed Virgin Mary + the saints, is already in the clutches of the devil.
Why would the devil need to lead astray those who are already astray?
You are espousing the heresy known as eternal justification which falsely teaches that men are born justified. The truth instead is that men come into a state of justification at some point in the course of their lives. It is during the interim in which the devil with great enmity is trying to stop the gospel from reaching them. This is the enmity the Devil has toward man from the beginning in Genesis 3:
"And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of
thy life: And I will put enmity between thee and the woman, and between thy seed and her seed"