What would Satan gain by tempting Adam?

Jesus Christ said of Satan: “That one was a manslayer when he began, and he did not stand fast in the truth, because truth is not in him.” (John 8:44; 1John 3:8) Jesus here shows that Satan was once in the truth, but forsook it. Beginning with his first overt act in turning Adam and Eve away from God, he was a manslayer, for he thereby brought about the death of Adam and Eve, which, in turn, brought sin and death to their offspring. (Romans 5:12) By his challenge of God and his charging God’s servants with lack of integrity, Satan lived up to his title “Devil,” meaning “Slanderer,” which title he deserved for having slandered Jehovah God in the garden of Eden.

Satan wanted to be a ruler as well. A position that did not belong to him. After his baptism, Satan approached Jesus in the wilderness with three different strong temptations, thoroughly testing him on the issue of devotion to Jehovah. In one of his appeals Satan showed Jesus all the kingdoms of the world, claiming them to be his own.

Satan became the ruler of this world when he was cast out of heaven. As mentioned at Revelation 12:9.

With each passing year, the world is becoming more and more dangerous. It is overrun with warring armies, dishonest politicians, hypocritical religious leaders, and hardened criminals. The world as a whole is beyond reform. The Bible reveals that the time is near when Jehovah God will eliminate the wicked world during his war of Armageddon. This will make way for a righteous new world.​—Revelation 16:14-16.

(New World Translation of the Holy Scriptures)