Why would God, being all knowing, create a soul that would he knew would become the devil? You probably don’t know the answer because I don’t think anyone does but do you have a theory?
You’re right. I don’t know.
I try to mostly stay away from theories as well. Sure, I could come up with a bunch of things. But they would be no more or less valid than anyone’s theory.
What I do want to address, for consideration as it relates to your question, are two ideas.
1. There must needs be opposition in all thing (2 Nephi 2:11). If there were no Satan, where would opposition come from?
2. We all have agency. One could, theoretically, ask the same question concerning all who choose evil. With God’s foreknowledge, why would He create those who He knew would never achieve exaltation? As with your question, we do not know. What we do know, however, is that God’s foreknowledge does not remove our agency. It is our choice to do as we will. It was also Satan’s choice to do as he did. God gives us all our agency in spite of His foreknowledge. We are free to choose.
We cannot possibly understand the meaning behind all things. And God has not revealed the answers to questions like these. But we can understand that sometimes, through their agency, others can be opposition to our efforts to return to God, and that we all may choose our own destinies in the eternities.