"There were plenty of Europeans who 'went Turk' and became pirates."

My understanding (which may be flawed) is that the term 'went Turk' or 'turned Turk' indicates that the party in question converted to Islam (usually under duress), as was not uncommon among Europeans who were captured by the Barbary pirates and forced into slavery. Some such converts no doubt became pirates, but there were probably seamen who joined the Barbary pirates without converting, and others who retained their autonomy while forming alliances or 'understandings' with their Barbary counterparts.