Mercenaries have been around, in one form or another, for ages.
But getting at what, exactly, a mercenary is and how it's different than your typical legitimate soldier turns out to be tricky. Our intuitions might drive you to say that a mercenary is anyone who wages or participates in war primarily for the remuneration she'll receive for that service. But that definition won't work. It won't work because we can think of someone we'd want to call a mercenary who factors in ideologies when deciding who to work for (for instance, no matter how much you might get paid, you might not be willing to work as a mercenary for someone fighting your own country). And, it could also be the case that someone working as a traditional soldier in a legitimate army is only doing so because of the pay; after all, military service is very often boasted as a career opportunity. So the idea of what a mercenary is can't be based on being paid for military services.
So I'm not sure we can get a completely clear and unproblematic conception of what a mercenary is. Nevertheless I think we can work with the idea that a mercenary is someone who looks out for herself, or her interests, while working outside the legitimate traditional institutions of soldiering (even if that involves working for a legitimate traditional institution of soldiering). I realize there's vagueness in that, but I think it's – coupled with the intuitions backing up the initial attempted definition in the preceding paragraph – clear enough to work with for now.
While you might think that mercenaries are only common is poor lil undeveloped countries, war-torn regions, or your mom's bedroom, they're really all over. Recently, and by that I mean in the last few decades, a new form of mercenarism has develop: the private military industry. These kinds of mercenaries are everywhere, but they're typically based in wealthy developed nations. This industry isn't the same as the military industrial complex (which is the business of making arms for the military). Rather, the private military industry is the business of providing “security services"; The private military industry is the business sector comprised of firms whose primary purpose is to offer services (in one form or another) of the variety of what legitimate state militaries traditionally (or theoretically could) provide. So this includes everything from highly trained combat personnel to logistical support. At the end of the day these firms are the modern variant of mercenarism; they offer the same kinds of services offered by “traditional” mercenaries, but they do so via a contemporary international business model; they are corporatized mercenaries. However, while you distill it, this difference doesn't amount to much of a difference at all: we can't meaningfully (or, at least, morally) distinguish them from other kinds of mercenaries in virtue of the fact that those working for a private military industry firm collect themselves under a corporate banner.
Now, having said all that, what's the problem? I mean, sure, most people seem to have the intuition that there is something morally repugnant when it comes to mercenaries: that waging war through mercenaries is, in some way, the wrong way to wage in war; or that the reasons someone might be motivated to act as a mercenary are the wrong reasons to participate in war. But are our intuitions here right? Even if we assume some sort of Just War Theory doctrine, our intuitions about mercenaries still seem indefensible. For instance mercenarism can't be bad because mercenaries do it for the money since soldiers in the regular army could, just the same, do it for nothing but money (and we don't want to say they're on the same moral standing as mercenaries). Nor can we say that mercenarism is bad because mercenaries are necessarily looking out for themselves; a mercenary could be just as motivated by a legitimate cause as a freedom fighter or patriotic soldier. And the fact that they operate outside legitimate traditional institutions of soldiering seems irrelevant as well – we want to say that freedom fighters, civil war fighters, and the like aren't also necessarily immoral because they, like mercenaries, are outside legitimate traditional institutions of soldiering. So what's the unique problem with mercenaries?
Well, in the end I'm not sure there is one. But if there is one, it might be grounded in how we conceptualize mercenaries - that is, that which makes them unique from other kinds of partciapants in war might be the same grounds for why they're morally repugnant. So, if there is any case against mercenarism, that might be where we need to look (or at least start). In the end, though, I'm not sure how fruitful that would be. In the end, our intuitions here might just be wrong. It could be the case that, most often, mercenaries are motivated by the wrong reasons. Yet that's not enough to say mercenarism is necessarily immoral and never okay. I could say more, but I think that’s enough for now. This is just my spur-of-the-moment thoughts on the subject. I could be wrong. After all, what do I know.