There are many answers to this question. Be sure that the following answer I will be writing here would not be the only one.
Okay, why did God create Satan knowing that he would rebel?
First of all, we must know that when God created Satan, He created him in an environment and situation wherein there was every reason to love God and be faithful to Him. That there was absolutely no justifiable reason to go against God, that, if he would rebel, it would be out of inexcusable wickedness on his part.
With that in mind, the reason why God created Satan knowing full well that he would rebel against Him is that Satan would become an object lesson to the other angels, who would not follow Satan in his rebellion, to remain faithful to Him for all eternity.
In Satan and his fallen angels, they would see and realize the folly, the consequence and the cost of what it is to rebel against God. They would be wiser, not from their own follies, but through the folly of others, namely, Satan and his fallen angels.
