There is no dark- and light-side; It's Dark and Dim side. One is a description of the rooms the cleverer side of the Corps work in, the other relates to the duller wattage of the brain of the other side of the Corps!
Darksiders are mostly, but not all, intelligence bods doing real analysis using clever capabilities and producing verifiable information that is actually useful and valued by the recipients. They quickly get disenchanted with the general amateurism and incompetence surrounding them, are passed over and leave to get paid more as productive members of society. Lightsiders are mostly, but not all, intelligence bods making shit up using PowerPoint or coloured pens and acetate, producing opinion-based fact in tediously obvious assessments that are ignored by the recipients. They quickly get indoctrinated to believe they are masters of the world, are promoted well beyond their actual intellectual capability and eventually leave to get paid more as management consultants.
Nobody really uses the terms Darksider or Lightsider anymore, outside of ARRSE or Cold War reunions.
PS And please never, ever again say "people" at the end of an opening sentence. I know you saw that film with soldiers in when you were twelve which made you think it was sweet, but in case it has escaped your notice or you are still twelve, many things which were sweet when you were twelve like kiss-chase or Crocs or that boy band you liked are not actually sweet, but are in fact sexual assault, permanent blisters and a miserable life spent repressing your apparent homosexuality.