Hmmm... that's a slippery question. To believe in the devil as an entity, I would first have to believe in god as an entity, and I don't. I believe in an energy that binds humanity together and tugs at our consciousness, that gives rise to religions in attempts to explain it. That draws people together in communities and in churches and in festivals. And just like with light and dark, I can't fathom how it could exist without a polar opposite. An actual conscious entity though? That I doubt.
I think you're on an interesting path with the idea of that ''binding'' energy...
Atoms get bound together to create molecules, which are bound together to create complex molecular structures like DNA strands... and thus ever greater structures emerge out of the union of smaller ones... You end up with creatures like humans who can communicate thoughts like these via digital technology... This 'binding' energy is obviously a fundamental part of the world we live in...
This binding energy also expresses itself in our social interactions... We all have a drive to relate to one another, to unite into a greater whole (tribes, society, nations, families etc.).
Love is perhaps the ultimate form of this binding energy and it is no wonder that religions, when they are at their best, focus on this aspect of life.
But as you said, it's hard to imagine this binding energy without its polar opposite. We could call it the ''dividing principle''...
There is life, but there is also death... When we die, our body loses its cohesive unity and its global structure breaks down... It disintegrates... Everything goes back to the sub-structures (molecules... atoms...) It goes back to the earth...
Socially, we also have a drive to seperate ourselves from the mass... to be our own persons. Without this 'divisive' principle, individuality could not exist... Communism seems to have failed mostly because of its failure to aknowledge the need for individual freedom. The need to stick out from the mass (for better or for worse)...
It seems to me that these two opposite principles (binding unity vs. multiplicity) are one of the pillars of reality as we can understand it. Everything can eventually be reduced to an interplay between these two fundamental principles... Yin and Yang comes to mind...
This inherent duality is ever present in our lives. And it's tempting to associate good and evil to all this... ''Good'' seems to be what unites us all while ''Evil'' seems to be what divides us.
Yet, I have a hard time imagining how rape and torture can fit in this delicate cosmic balance. Aren't some things just plain BAD?!!!
There's a difference between establishing frontiers and barriers between you and me and downright hating each other wanting the other to suffer...