I believe devils do exist. If we took our history with seriousness, there is no shortages of devils and monsters there which makes history so depressing and traumatizing. If you are in the field of criminal psychology, the markers between monsters and devils are almost identical its existence is unquestionable. It is a good thing though that our ancestors introduced us to the devils like those you mentioned because the intent is that anyone can turn out easily to be one if you are not conscious about it. I say not in a literal sense but the general idea that if you become evil (like the devil), the world you will live in will be dark, malevolent, and unpleasantly meaningless and die in vain. Although some find it meaningful, which is scary.
The devil, in my understanding, is based on the idea of extreme negative human behaviors to which our ancestors were able to discern since the birth of human consciousness. They make stories about it and passed it from generations to generations to understand its reality and existence. There's no science back then so our ancestors created stories after stories with their own imaginations to deliver the idea. One good example is the stories in the Genesis you mentioned which is a narrative that is so profound and deep that even in the modern times, it is still relatable, discussed, and studied.
I say that the modern people are disassociating themselves to profound religious ideas, in which the human civilization was founded, due to polarized human politics and arrogance to the technological revolution. It costs us now to tilt more in to chaos. The question of whether God created the devil is becoming of less significance now. Today, the important question is how are we going to create a new devil in this era. There is no doubt that mankind had produced and created many devils in the past, so it's not suprising to me that we end up creating another who are far more powerful than ever with mankind's advanced sophisticated technology as its arsenal. As pessimistic as I can be, until we understand the significant wisdom and profound knowledge from our own history, we inevitably bound to repeat it.
My personal stand on someone who believed that God created everything including all the bad things here and the primary force why mankind suffers is a contempt to the ideal Being, irresponsible, and naive without even questioning one's proclivity to do evil and ignoring to point out that it is indeed evil. One has to look up in the mirror and ask themselves 'have I live up to my own ideal?'.
