Tough as nails, and just as painful should you step on him, Mark Hazzard is a gun for hire, a mercenary. However, Mark couldn't simply work for anybody, as his own principles prevented him from doing soul-less dirty work for evil men. When he was on the job, and his employers turned out to be such, Mark invariably put them in their place, often by killing them outright.
While this caused no end of frustration for the other fellows in Mark's mercenary crew, he slowly started to rub off on them, and over time, Mark profoundly affected the working habits and scruples of those that fought with him - of course, he had to shoot one or two of them to make them see the light. However, as true friends, they got over this, and came to love the man.
This is why, when Hazzard got set up by his ex-wife's new husband in Iran, all of his combative friends got together and went to rescue him - they'd realized that there was more to life than mere money. In fact, after they saved him from this mess, many of them went on to train freedom fighters in the art of battle, though Hazzard himself didn't live to see this.