Heidi

Heidi:
I've found that when you're a mercenary people assume it's another word for assassin. Don't get me wrong, I've killed people -- lots of people. It's what I'm trained to do. However, if I kill there's a reason for it. I don't kill just for the hell of it and I never kill just because someone wants another person out of their way.

I had to read the first sentence a couple times because "you're" after "I found" sounded to me like the narrator was accusing someone of being a mercenary. See if this is clearer?
When people hear I'm a mercenary, they assume it's another word for assassin.

That is, let's get straight that I the narrator is the mercenary-- there's no "you". Now if you want the narrator distancing... but no, immediately afterwards he/she says, "I've killed," so that sort of admission means a certain level of candor.

When people hear I'm a mercenary, they assume it's another word for assassin.
Don't get me wrong, I've killed people -- lots of people.

From the first sentence to the second, there's some transition missing. Yes, they're right, I am an assassin. No, they're wrong, and here's why. You're stating the conventional wisdom in line 1. The rest of the paragraph should be an explanation of reality-- but you probably should say that the conventional wisdom is wrong. Like:
They're right, I do some killing. But I have a code. I don't just kill because I'm paid.... or whatever.

See, the problem is, you're just presenting that he/she is indeed an assassin, that the conventional wisdom is right. Distinction without a difference, maybe? It's not like we have that great an idea what an assassin does. Quick. Name an assassin. Lee Harvey Oswald is the one that comes to mind for me. And he didn't (presumably) kill just for money. I think "hired assassin" or "hit man" is more what you mean. We are far more likely to think of an assassin as killing because he's crazy or for political reasons than for money-- at least I am, because "assassin" is linked to political assassinations. Well, for me anyway. "Hired killer" would be perhaps my third definition of that. "Mercenary" is much more associated with earning money as a free-lance soldier, so is more, uh, mercenary.

Anyway, that whole distinction you're presenting in the first few lines is more confusing to me than illuminating. (This might just be me.)

I've killed people -- lots of people. It's what I'm trained to do. However, if I kill there's a reason for it. I don't kill just for the hell of it and I never kill just because someone wants another person out of their way.

Too many "people" (also in the first line, presumably a different set of people than the ones who get killed). This is sort of slack-- notice the "person" in the last line, and "someone" (singular) replaced with "they". First person doesn't mean that anything goes. Go sharper, not duller, with the voice. This is a hit man. Sorry, a mercenary. :) He/she admits to killing. Is he/she going to talk wimpy or tough? Non-specific is wimpy. Specific is tough:
When people hear I'm a mercenary, they assume it's another word for assassin.
Don't get me wrong, I've killed people -- lots of people

"People" is slack. "Victims" is cold-blooded and direct.
"Someone" is slack. "Client" shows the business relationship.
"Person" is slack. "Rival" (or whatever) show the "why" and is specific.

Let's try that from the top:

When people hear I'm a mercenary, they assume it's another word for hitman.
Don't get me wrong. (Note-- the comma makes this a comma splice, which isn't necessary-- never make unnecessary grammar errors, because I'll just have to fix them in an edit, and we will have irritated each other to no purpose. The more clipped period there will match the voice better, I think.) I've killed. I've had lots of victims. It's what I'm trained to do. However, if I kill, there's a reason for it. I don't kill just for the hell of it, and I never kill just because a client wants a rival out of the way.

As I said, I'm particularly muddy-brained this evening. But challenge yourself to find precise nouns, and just see if they sound better.

A