I'm thinking of building a world by mixing two existing worlds: Dexter and Stargate SG-1.
In Dexter, the main character is an evil psychopath/sociopath (note: I'm still not 100% what the difference is, but it is immaterial to this question).
In Stargate SG-1 (at the beginning), the villains are the Goa'Uld who inhabit and control the bodies of helpless humans who are then forced to watch [almost] forever while their bodies are used to commit horrible atrocities.
It's well-known that the Goa'Uld are able to relinquish control to their hosts - but why would they? They're bent on world domination.
Until, of course, they inhabit Dexter, thereby creating host and symbioses each as sadistic as the other.
With the power of two equally sadistic minds working 100% in tandem, the resultant entity would be almost entirely unpredictable.
Societies today, during warfare, attempt to draw distinct lines between the self and the enemy because that allows you to write them off as "not-self" and therefore not worthy of the protections you would otherwise afford the self. When the lines become blurred, when you can no longer determine if an act was that of a human committing a crime or an enemy committing an act of war, you ability to fight diminishes. We, to some extent, see this with the war on terror because the "enemy" is much less defined than it usually is.
What would be the societal impact of the combination of the self (humans/Dexter) and the other (the Goa'Uld)?