Mercenaries: Can be useful in dirty jobs? Not really because if the Big Authority, the only one which has the legal right to use violence, gives the Green Light, some special squad would do the jobs…
The only use of mercenaries is to hind the casualties…

You do not join the Army or sign for modern mercenaries for the same reasons. One is for ideological and the second for money (Iraq), boredom, need of adrenaline and others…

Then what laws apply to mercenaries? Geneva is caught prisoner, Common law if they kill some body? Or no law at all, random law depending the case, the employer, circumstances etc…
What if they change side? Mercenaries can do this. You can change employers, giving your notice then do to the other side… Would be it betraying? Or just usual and common employment rules? In some jobs you can’t apply to a similar post in concurrent companies for a certain amount of time (e.g. if you know clients).