"Military contractor" is really just a euphemism for mercenary. "Mercenary," in turn, is just a euphemism for pirate.
Companies like Blackwater (now Xe) are not motivated by patriotism or duty or honor. They practice war for money and money alone. As long as the United States is the highest bidder we can count on what passes for loyalty in their black hearts. But if drug warlords in Afghanistan can outbid the United States, as is apparently what has happened, then Blackwater mercs have no qualms switching sides for greater profit.
However, given military contractors have been accused of murder, rape, running drugs and guns, and prostitution I think it is safe to say that these mercs don't have any qualms about anything whatsoever.
Mother Jones put it all together a couple of years ago.