Satan is an amalgamation of the evil that strikes mankind's collective heart. In the same way that God is the good.
Is Satan real?
>no
Is the idea of Satan real?
>yes
Humans have the ability to obfuscate and socialize. We've removed ourselves from the natural world and have subjected ourselves to social selection. Every culture creates a meta-story that describes a collective of heros or anti heros that exist over time. These are stories that tell the best way to live to further the culture. They also describe the evil of the world.
An example: a warrior fights for his people and is successful in defending them. He is admirable, competent, and heroic. Tell a story about him. 10 years later a different warrior does the same. Tell a story about him. Do this for thousands of years and what evolves is a meta-warrior hero that encompasses the important traits to the greater culture. A story that describes the ideal warrior. A story that's derived from the success of thousands of ancestors that came before.
The personification of this story is a god of war. The beginning of polytheism. Do the same for love, for hate, for nature, for light, for evil…Anything important to a culture that ensures it thrives and will be socially fit. It's a defensive mechanism humans have evolved that allows their future descendants to reference the past stories to revivify the culture into the future. To adhere to tradition in order to ensure reproductive success.
These gods then begin to fight and over time a meta-metagod emerges and this is the idea of a single god. The same thing happened with Satan. You can be benevolent or malevolent. You can follow the right hand path or the left hand path. You can adhere to a life of light or darkness. This polarity is represented in symbols like the ying-yang. There's always darkness in man, even when the most enlightened. There's also always a spark of light in evil.
Here's the thing: the old testament described a tribe that must contend as a group against other tribes. It's ultimately about the sacrifice of the individual for the benefit of the tribe. This is why Judaism is fundamentally centralist and controlling. Their religious text, their amalgamation of historic stories, tells them that this is the way to survive.
The west was predicated on an evolution of that idea: the responsible individual who shoulders their suffering and moves up the hill, despite the chaos and malevolence of it all, is the way to succeed in revivifying the culture into the future. It is LOVE that makes this way of life meaningful and carries the good into the future.
This occurs with evil. But evil exists through corruption and control. That's the only way it can maintain continuity into the future. They're subjected to the same selective social forces as the greater culture of good. Evil corrupts the individual's ability to decide. It controls those decisions through temptation, fear, or torture. Those in the Cabal are all subjugated to mind control at a young age as it's the only way to carry on the spark of evil into the future. Capturing someone in blackmail or willingly submitting to a "deal with the devil" for power/lust/greed is all forms of corruption of the greater culture.
This is a deep subject and there's too many threads to pursue. The gist of it is this: Mankind is responsible for all the good things in the world. Ultimately we personify that in the image of a deity. We are also responsible for the most horrific things that mankind is capable of. We are ALL capable of atrocities. This is personified in the image of a deity as well. "Does the idea of satan exist?"