Mercenaries were highly mobile in Renaissance Italy, moving from contract to contract. Professional captains called Condottieri, usually paid for by cities, recruited men for a fixed amount of time. Many of the for-hire soldiers who served in Italy weren't even Italian, coming from places like Germany, Switzerland and Hungary.
In their perpetual struggle to dominate Italy, the Italian states hired mercenaries to wage war against their Italian neighbours, and the soldiers, many of whom had no code of honour, would often rape and sack freely.
When the state that was bankrolling them ran out of money, the soldiers would disband, frequently to be hired to destroy the very city they had just fought to defend.
Some mercenaries were honourable, fighting for a cause they believed in, but men such as these were few and far between. Where they existed, however, they were much sought after.