I think the real question is why there are mercenaries. For this many mercenary groups to exist there would have to be something inspiring and encouraging it. That implies many fights happening between many tiny nations. In fact this world would work very best if you had no super powers and only lots and lots of tiny nations, all so small that it's too expensive to keep a standing army and it's easier to buy mercenaries for the length of time you need them.
The concept of 'buyability' would then be very important. If I have to hand over a good portion of my nations GDP to buy mercenaries to defend it I would be rather upset if the mercenaries then turned around and ransacked it. Thus most mercenaries would have a very high value on keeping their word. Those that are known to break it would never be hired, or not by anyone remotely trustworthy.
Since mercenaries are so much stronger, and thus nations probably keep little in terms of standing army, the threat of mercenaries simply moving in and taking over a nation is very high. Of course any sort of group that does this is unlikely to have the skills to lead and keep it strong. Thus some mercenaries may move from country to country, taking it over, using it's resources and then moving to the next Country.
This all combines to give a pretty interesting premise for the world. Mercenary corp X was hired to defend a nation but the nations enemies gave them a huge bribe to take the nation out instead. The corp agrees to do so, only to discover that, shockingly, no one will hire them ever again. So they try stay in the nation they were originally hired to defend and simply live off it's resources, but they find the nation is small and without good leadership (which they killed) it starts to fall apart into anarchy fast, they don't want the difficulty of trying to lead, and so they move on leaving the collapsing former nation behind in anarchy.
X proceeds to move from country to country, taking out their tiny armies, enjoying it's riches, and moving on when the company inevitably collapses from the lost of leadership and raping of it's resources. Neighboring countries, which formerly didn't bother with large standing armies, are now terrified of being invaded by X and effectively destroyed. They start hiring mercenaries to defend themselves, and suddenly all the mercenary groups around the world are moving in to a small area full of tiny disorganized nations afraid of being invaded.
The suddenly influx of so much man power, and such drastic expenditures to afford the mercenaries, then causes odd power disputes. Country Y hires expensive mercenaries because their afraid X will invade them. X hasn't invaded yet, but Y realizes they can't afford to keep paying it's mercenaries to stick around and defend them without eventually going bankrupt. Y decides since their on retainer any way they should use the mercenaries to help cover their own cost and invades undefended nation Z and taking their resources. Z has to hire mercenaries to defend itself and suddenly Z and Y are at war. Some country bordering Z and Y then hires mercenaries simply out of fear that either Z or Y will attack through them to get to their enemy etc etc. Meanwhile none of these nations have the GDP to be able to keep up with the massive expense of keeping these mercenaries on retainer to defend them, the nations are all slowly bankrupting themselves to keep the protection. In the long run even if X never invades the nations risk collapsing when their markets can no longer sustain the massive mercenary costs. It's an unstable and bad situation which looks to get far worse the longer that X is out there terrorizing the nations.
In all this you have X as the evil mercenary group to be your antagonist. You have wars suddenly happening all over the place, political nuance (you can glance over it or mock it if its not relevant to your story, but leave it implied in the back story). You have something inspiring all the combat and warfare, and it being bloody and to the death because all the nations feel that their fighting for their very survival and people can be crazy in a mob mentality like that.
Your protagonist can be the heroes simply by always keeping their word. Perhaps mercenaries have codes for how they work together. In either case the protagonists ultimate goal is to stop group X. In so doing they can fix the whole warfare nonsense that X accidentally spawned. You get gore and bloodshed while being the good guys, yay!