In the original sin of Adam and Eve, Satan held out to them the promise of attainments beyond what they had experienced to that point in the Garden of Eden. Tempting them, he said, "You will be like God, knowing good and evil" (Genesis 3:5). This helped motivate them to become greater than what God, their Creator, had assigned to them.
In Genesis 1:31, when God saw everything He had made, He declared it to be "very good." Pride is not "very good." It was not in them as God created them, but it entered into their thinking in Satan's presence. The very first exercise of that pride earned them death and ejection from God's presence and the Garden.
At some time following his creation by God, pride arose in Satan, and he uttered this desire, as written in Isaiah 14:13-14:
For you have said in your heart: "I will ascend into heaven, I will exalt my throne above the stars of God; I will also sit on the mount of the congregation on the farthest sides of the north; I will ascend above the heights of the clouds, I will be like the Most High."
His devious offer to Adam and Eve in the Garden is an echo of his challenge against God. This exercise of his pride separated him from God.
Pride, Satan's lofty feelings of superiority regarding his beauty, corrupted him. It deceived him into wanting even greater power to complement his splendor. After all, he deserved it, did he not? Notice how great he was in his own eyes!