Jean-Baptiste Bernadotte/ Charles XIV John of Sweden (One of Napoleon I's Marshal) betray his nation?

I don't know whether he betray his country of not, but he did allied his kingdom of Sweden and Norway with Britain, Austria, Prussia and Russia during the end of the Napoleonic war against his origin country, France. Was this counted as a betrayal? How does he elected from a humble French Marshal into a Swedish King? And why he did to his own country like that?