First of all, both angels and humans have been given free will. Angels however are very different from us. They are pure spirits, without a corporeal body and so does not experience physical change. And with no physical change they are not bound by a continuous timeline as we do. So although both angels and we can choose, we choose from one moment to another, and without being able to see beyond the present. Yet angels being outside of time can take in entire causal chains from this moment down to eternity. Their only limitation is their creatureliness, they cannot hold all things at once. But when they do focus on one thing, they can see down to eternity how their choice affects everything.
So you can imagine. God indeed created them good, and truly free. But because they truly know all the ramifications for every decision they make, every decision they made is final. It is final not because they are not allowed to change them later; rather it is because when they made a decision, they are not just making it for this moment, they actually make it for every point of time from this moment down the causal chain to eternity.
So when Lucifer chose to rebel, he did not simply choose to rebel God back then in time, but that one decision, being out of time, applies instantaneously at every point of time for eternity. He has already chosen never to repent, knowing and seeing fully all the consequences it entails throughout eternity.
Why did God create such power? That’s the same question as why did God create you and I. Without such awful freedom, there is also no sharing in the beautiful love that God is. It is precisely because God’s glory and love is so amazingly good, that when it is twisted it is so terrible. But God still chose to create because He believes it is worth the risk, that love is stronger than evil, and that more will come to choose love freely, and ultimately evil cannot harm those who are in God, for every time one chooses God, he belongs to God and His salvation is at hand. As powerful as Satan is, he is a creature and a defeated one before God. He can lure and harass and threaten, but he has absolutely no power against God and those who cling to God. He is a barking dog chained to a pole. Unless we foolishly choose to believe him and walk into his domain, he is harmless and his threats are futile.
But he knows us better than we do, and he knows all our weaknesses. So be on guard and look to God in all things big and small, and give no provision whatsoever to sin or temptation. Run to His mercy when we stand or when we fall. Trust in His power to save. Then however powerful evil may seem, it is merely a temporary reality that will pass with this world.