The comments on the top voted answer called for more elaboration, so here I go:
"Power resides where men believe it resides. No more and no less"
The king holds the power if you and the sellsword believe that you and your family will be hunted to death after you killed the king.
The priest holds the power if you and the sellsword believe going against his word will result in eternal damnation.
The merchant holds the power if you and the sellsword believe he will offer riches and save you from repercussions.
The sellsword holds the power if you and the sellsword believe the three men are depraved lunatics that deserve to be killed and looted.
Nobody holds the power if the sellsword believes life is meaningless.
The keyword is "believe" - these things need not be true, but they need to be something the sellsword believes to be true. There are a hundred other beliefs the sellsword could have, all (or at least most) of which would lead to either one of the above results.
Or in other words: The moral of the story is, if you are the king/priest/merchant and the people around you don't believe you hold power, you're dead.
Another possible answer, although not the official one, is Fate (aka GRR Martin). From the context in the book however, this wasn't the intended interpretion by Varys.