Well the idea of Spartans goes like this:
Basically, they're guns for hire - who occassionally do the gun thing without being hired (when short on cash). What separates them from common bandits is their military organisation and philosophy, summed up in:
- Loyalty to their holy book, the Rules of Engagement (a distorted caricature of chivalry);
- Right to bear arms and personal liberty;
- Survival of the fittest;
- Genetic and ideological purity (survival of the Spartans themselves);
- Cookies, I mean, those of their females who aren't considered pure enough for breeding (too smart, too mouthy, too independent etc), are disallowed to say "no" to anyone (hence they sometimes fight on the frontline in the hopes of escaping this status through strength and wealth).
They despise all current forms of governance, from the Stellar Empire to the Provincial Govts to the Mutant Alliance, having a catch-all term for all of these, "Socialists".
They live in hidden forts and bunkers in the wild (the survival part!) and recruit from all walks of life, with the young recruits being put through the gruelling experience of frontline combat (and being allowed to keep up to 10% of the pay/loot). Those who survive long enough, and distinguish themselves in the eyes of the Officer Corps, are allowed to join it. This is a motivation enough for many to join the Spartans, since the Officer Corps rarely risk their necks, while keeping said 90% of wealth "for war effort".