If your question is serious then I can give you Muslim perspective according to what I know.
Devil or Satan’s real name according to Quran is “Iblis”. He was not created as an angel because angel do not go against Gods will. He was one of the “Gins” Now word “Gin” always gets confused with “ghost” in English language due to the restriction. According to Islam “Gin” are another creation of God but have been created from fire as compared to human created from clay, mud etc. Most important thing to note is that Human and Gins have been given free will unlike angels.
Now Iblis had been given a rank somewhat equal to angels due to his worshipping of God. Until God created first human, Adam (pbuh) in this case and God asked all of the angels and Gins to prostate to Adam (pbuh). All of them did apart from Iblis for the reason he believe that he was far superior for being created from fire and that human will create problems (as he believed) . For that reason God stripped him from his high rank and decided to punish him but to prove that by creating human God had not committed any mistake let him free on his request to allow him time to prove his (Devil’s) point. God allowed him time and said that go and prove you are right but you never would be able to target people who believe in me and do as I tell them to do and I know what you don’t.
Apart from that story is long and has been given in much detail in Quran.
Maybe, just maybe, you are all mad. God, the devil, they are all in your own minds. God is someone you pray to when things are not going right, or you are about to die. The devil on the other hand, is someone that is here in everyday life, when wrong decisions are made.
So if an angel goes against God's will then they must be redefined as they were clearly not an angel in the first place. Either that or angels are too scared or incapable to do otherwise. Hardly a heavenly place as most would picture it.
It need not be a design flaw. It may be an inevitable risk in order to achieve something else more important.
Lucifer was one of the great angels in Heaven. But he got too big for his britches and rebelled against God.
God couldn't let him and his forces stay in Heaven, so he invented Hell for them as a place where they could suffer unspeakable torment while they considered what a misfortune it is to be on the losing side.
No free will for angels eh. Just automatons, incapable of knowing and chosing. Well there's a turn up for the book. Still I have no proof anyone I meet is any different I suppose.
Ah that's an interesting viewpoint. So a presumably compassionate deity creates a creation that had no say in the matter, gives it the ability to opt to do things the deity disapproves of, and then deliberates creates a place where they could suffer unspeakable torment. Sounds positively evil to me.