Private Military Contractors/Headscratchers

In the modern era, mercenaries make for squeaky-clean villains because they're convenient. Don't want to worry about pissing off a particular nationality? Have you bad guys' mooks be from a thugs-for-hire mercenary group. It doesn't help that companies like Blackwater got massively bad press for their actions which further makes PMCs look bad. It also doesn't help that, like was noted above, it is easy to portray people who kill and fight for only money as villains because they don't have a "cause" beyond greed. Even the United Nations agrees with this, as mercenaries are legally considered unlawful combatants and do not get POW status. That doesn't mean all mercenaries are untrustworthy, sociopathic villains, but it does make it easier for them to be used as villains.
 * I've noticed a trend in a lot of media for mercenaries to be portrayed as almost universally despised as untrustworthy and amoral--A Song of Ice and Fire and Fire Emblem come to mind. But where did this idea come from? Historically mercenaries were highly valued, from Swiss pikemen to Genoese crossbowmen, and they're widely used today as well. Why do writers consistently portray potential employers as being extremely skeptical of hiring mercenaries? In both the examples I mentioned the mercenaries are heroic, or at least less villainous, protagonists, which makes it especially strange that they would be functioning at all if everyone was so biased against their existence.
 * The way I see it, it works like this in common perception: People who are willing to kill other people for your cause out of conviction, loyalty, or duty are valuable allies, while people who are willing to kill other people for your cause solely because you pay them to do so are merely hired thugs. As for mercenaries being employed at all even where nobody likes them, remember that mercs do have a few practical advantages over regular troops from the employer's point of view -- mainly that they tend to come already trained and more often than not equipped, which may actually save money in the short run and definitely does save time if you need more warm bodies on the battlefield on short notice, and that as 'outsiders' they're considerably more expendable if need be as well.
 * Part of the reason why free companies were valued in medieval and later eras was because they were frequently the best source of professional troops one could ask for. At the same time they were also untrustworthy, because once the ruler who was using them ran out of money, the free companies would run off and do their own thing. Free companies that weren't being used as soldiers in some king or lord's army were frequently brigands who tended to rob and rape their way across the countryside, and they couldn't be stopped. It got so bad in France that the French crown tried to destroy the free companies by forming a royal army of feudal troops, and the free companies united and kicked their asses. The French only dealt with the problem by hiring the free companies for campaigns abroad.
 * The Thirty Years War. While the national armies were just as bad, and some far worse, mercenaries got a lot of the blame for the atrocities committed.
 * Context is important. A mercenary in a setting filled with conflict can be portrayed as a good guy but in a peaceful realm, "mercenary" is a euphemism for "assassin", therefore the people who hire them are looking to have someone offed. Not all mercs are bad; Michael Weston from Burn Notice is a good guy mercenary, as is Chris Chance from Human Target. The attitude of mistrust boils down to them being willing to do anything for a buck, including leaping over the Moral Event Horizon or selling out their employer if someone offers a better deal.
 * Well, Machiavelli thought that mercenaries were unreliable and untrustworthy. He wrote in The Prince that mercenaries would abandon their employers when they get into too much trouble, or would even betray their employers if the other side offered more money. It was better to gather soldiers who would feel a personal loyalty to you and your cause, since they'd be more inclined to stick with you if things got tough.
 * Mercenaries are politically neutral targets for authors to use as bad guys. That means there will be more negative depictions right off the bat. Add in the real political implications of a person willing to set aside ideals for money, and you have the easiest bad guy you could ever ask for. A good mercenary tends to be of the sort who's less willing to take money, more willing to stick it to the real bad guys in his own way.
 * They also have built in justifications for Psychos For Hire: real life mercenary companies almost exclusively recruit former military personnel to avoid spending money on training, meaning that particularly unscrupulous ones would have no problem hiring veterans dishonorably discharged due to "improper conduct" (or worse, actively seek them out since their lack of other opportunities would cause them to not be picky with their salary).
 * I have my own theory for why modern Private Military Contractors are universally portrayed as villains: to avoid offending real life militaries and creating plot holes in the story, particularly in regards to the US Army. Sometimes, you want mooks working for the Big Bad General Ripper who have all the cool toys and elite training of the US military, but don't want to offend current service members. Enter the Private Military Contractor.