There is however a universal principal that a soldier does not betray his vow. Ever.
European history provides quite a few examples when a professional mercenary's vow was broken by condottieri at will. Mamertines anyone? Machiavelli describes many such cases: desertion, joining forces with enemy, switching sides.
I vaguely remember mercenary betrayals taking place during wars of Charles the Bold, Duke of Burgundy against Lorraine and Suisse, and his death at Nancy.
Polish allies at times: Crimean Tatars and Cossacks were also prone to betrayal.
General Patrick Gordon, a venerated Scottish mercenary, after already switching sides from the Polish to the Muscovite one (honourably though), was almost given a deadly blow to his division during 1695 campaign against Turks, because a German engineer betrayed to Turks the weak points of the Muscovite lines.
So it looks that there is a difference between romantic theory and practice.