[Sailor Steve]

I have a slightly different take on all of this. But only slightly.

Soldier: Fights for his country, or for what he sees as a cause. These days a lot of kids are signing up because the army offers more money than it used to, but they aren't signing up to fight at all, they're signing up because of the promise of a good job. Sometimes when push comes to shove they refuse to go at all, which causes interesting complications but doesn't mean they're fighting for the money.

Mercenary: Is just what the name means - someone who fights for money. They like to fight, or think they do, and will join any army that will pay them. In the middle ages whole armies rented themselves out to whoever needed them at the moment. At that time it was considered an honorable profession.

Assassin: Is a murderer, not a soldier, and he murders for big bucks. He names his price and if it's paid he does the job. He's not a soldier and he doesn't like to fight, or to take risks. Of course most of the more famous assassins in history didn't do it for money, but for a belief, which means they only did it once, not as a profession.