As a human being I have wondered that myself.
But it must have been His plan?
Not that God MADE him evil but he gave the angels free will?
He then knew what Lucifer would do and through that, WE have free will.
I mean if there was no devil there would be no evil and we would all serve God.
God wants us to CHOOSE to serve Him, not force us!
It's all I can come up with~