Lucifer comitted the worst sin of all, he was prideful. He told God he would place himself above God, and all would bow to him.
This has been Lucifer's goal from the start, to place himself ABOVE God, to be LIKE God.
That is why the Watchers took wives from the human segment of society. Lucifer wanted to start his own race of beings which would be loyal to him.
Lucifer is very clear about his ambitions in his "I will..." speech quoted in Isaiah 14:13:
I will go up to the heavens,
I will raise my thrown above the stars of God, and
I will sit in the mount of meeting, in the sides of the north.
I will rise over the heights of the clouds:
I will be compared to the Most High.