Early on, Arnold served his country honorably, Leading the Capture of Fort Ticonderoga, Invasion of Canada, and the Battle of Saratoga, among others. In 1779, Arnold married Peggy Shippen, a high-level spy for the British government. In less than a year he would betray his country.
In 1780, Arnold attempted to give control of the Fort at West Point, which he commanded, to the British. His plans were revealed when his British contact Major John André was caught with incriminating papers. André was executed but Arnold escaped. He saw limited action serving the British in the remainder of the war, and eventually was called back to England. He returned to Canada and then went back to London where he died, hated by his betrayed country, the country that he switched sides to and with little left to his name.
"Benedict Arnold" has become a synonym for traitor in American English.