You get the 'Big If' award of the day. Here's another 'Big If' for you: what if God didn't know that Lucifer was going to rebel? The same question could be asked about Adam.

If there was a god there would have to be a devil, you cannot have one without the other. Nature will always find an equlibrium. Since men invented the gods they had to have some sort of anti-god for balance. Without the contrast, how would you know good from evil, at least in the religious sense?

i didnt know he did

It wanted someone to spar with.

God gave free will , and we choose what roads were going to take . Yes God knows the outcome of both of those roads but by free will we have a choice of what road we shall take Lucifer chose rebellion against God .Adam chose to ignore God's warning . And we chose everyday to follow God or go our own way .God knows the possible outcomes for us but we choose the ones were going to take .

I have no proof that either one exists.

Maybe God had this plan already made. For good reason God let this happen.

Jehovah God gave all his creation free will...so Satan chose wrongly...making himself an enemy of Jehovah....Sadly, a third of all the created angels in heaven also chose wrong...and these ones became the demons. (Revelation 8:12) The fourth angel blew his trumpet. And a third of the sun was struck and a third of the moon and a third of the stars, in order that a third of them might be darkened and the day might not have light for a third of it, and the night likewise. In this scripture, the stars are the angels. (Revelation 12:4) and its tail drags a third of the stars of heaven, and it hurled them down to the earth. And the dragon kept standing before the woman who was about to give birth, so that when she did give birth, it might devour her child.

There must be opposition in all things.

That makes no sense. Still, it is part of the con game. If God is omniscient, he knew the Devil would lead many of his creations into eternal punishment. Only an extremely sadistic deity would allow this. God is far worse than the Devil, for he created him knowing what he would do and allows him to lead millions of humans to eternal torture. Fear of Hell and enforcing guilt upon people motivates them to pay Christians churches 10% of their money. It is a crude con game. If a person can be logical, critical and objective, he laughs at such a goofy tale. Clergymen try to arouse hysteria, so that People cannot be logical, critical and objective, of course. If God is omniscient, he is the ultimate sadist who creates humans so that he can torture many or most of them forever. He was not omniscient in the first tales about him. He comes across as a clumsy ninny who is quite ineffectual as a creator.

you need to ask god that question

Just as light and darkness exist, God received all the glory when Lucifer rebelled in Heaven by his freewill. Logically, God uses sin to prove that sin is bad. John 1:5 "And the light shineth in darkness; and the darkness comprehended it not."

Who is Lucifer? If you’re referring to Satan, God created a perfect, intelligent angel. All angels were given free will, the ability to determine right from wrong and choose their own course. Satan means “resister” and devil means “slanderer”. God did not give his perfect angel that name! Those are adjectives that describe the characteristics of the spirit creature that chose to rebel against his Creator. His God-given name is unknown to us, satan the devil is not worthy of having his personal name written in the Holy Scriptures. John 8:44

Lucifer / Satan was created an angel and along with all the other angels was given free will...He chose to NOT be obedient to Jehovah god's standards...and so became ' the resistor' Resisting goodness. And is the current main cause of all the worlds problems... But he will be eliminated... (Revelation 6:2) And I saw, and look! a white horse, and the one seated on it had a bow; and a crown was given him, and he went out conquering and to complete his conquest. (Revelation 12:7-9) And war broke out in heaven: Miʹcha

This is a serious problem with religion. First off, if God exists, but does not interact with us in any material way, there is no way to understand his motives, and therefore, no logical explanation can be made to address your question. Secondly, if a devil exists, yet is completely undetectable to us, then we have no way of knowing that devil's name, let alone whether his name can even be pronounced with the sounds that humans can make, therefore there is no rational way to address this question. Thirdmost, since the stories in the Bible defy all rational thought, there is no way to reasonably address this question.

The same reason people created guns despite knowing all the killing that will happen and it's all they are made for. But they serve the purpose they are made for and not everyone is stupid enough to fall for it or die by it.

All of Jehovah’s works are perfect; he is not the author of unrighteousness; so he did not create anyone wicked. (Deut. 32:4 The Rock, perfect is his activity,+ For all his ways are justice.+ A God of faithfulness+ who is never unjust;+ Righteous and upright is he. The one who became Satan was originally a perfect spirit son of God. When saying that the Devil “did not stand fast in the truth,” Jesus indicated that at one time that one was “in the truth.” (John 8:44) 44 You are from your father the Devil, and you wish to do the desires of your father.+ That one was a murderer when he began,+ and he did not stand fast in the truth, because truth is not in him. When he speaks the lie, he speaks according to his own disposition, because he is a liar and the father of the lie. But, as is true of all of God’s intelligent creatures, this spirit son was endowed with free will. He abused his freedom of choice, allowed feelings of self-importance to develop in his heart, began to crave worship that belonged only to God, and so enticed Adam and Eve to listen to him rather than obey God. Thus by his course of action he made himself Satan, which means “adversary.”—Jas. 1:14, 15

It was part of His original plan for us to be tested.

Matthew 13:30 Let both grow together until the harvest. At that time I will tell the harvesters: First collect the weeds and tie them in bundles to be burned; then gather the wheat and bring it into my barn.'"

To give man a choice

Lucifer was one of God's favorite angels until he started to want to take over the place and do things his way that's when he was cast down to earth

Did God create Satan the Devil? No! An angel who had been in heaven when God prepared the earth for Adam and Eve changed and became the Devil. Job 38:4, 7 How was that possible? Well, how does an honest person change and become a thief? He was not born a thief. But he desires, or wants, something that doesn’t belong to him. He keeps thinking about it, and his wrong desire becomes stronger. Then, when he gets a chance, he steals it. He has turned himself into a thief.

i think he was an angel at first and there was some kafuffle over jealousy ..am not sure how correct that is

For the same reason that any of us have children and tell those kids stay out of the cookie jar until after dinner. You know darn well the minute that you tell somebody not to do something that's the first thing they're going to do. And you know in advance before you have children they're going to disobey you many times. I'm sure God knew the minute that he told Adam and Eve not to eat from the tree that they were going to do that. I can only assume because he's God he had a reason why he wanted them to go through everything the human race has gone through.