The betrayal done by the traitor is remembered for generations and never forgiven
Benedict Arnold: He was a great soldier for the American side of the Revolution to the point that he could have been remembered as an American hero, but he chose to betray America and join the British. For generations his name was, and sometimes still is, used as a synonym for a traitor in America.
Ephialtes: During the famous battle of 300 Spartans (the real number is higher, but that is the culturally known reference number) against the Persians at Thermopylae Ephialtes revealed the location of a path that the Persians could use to get around the Greek formation. The Greeks would place a bounty on his head that was honored even after he was killed for unrelated reasons. His name would become a word in the Greek language for nightmare. (Image from the 300 movie as I couldn't find an actual depiction of him, presumably due to him being a traitor no one wanted to make art of through history.)
Vidkun Quisling: He was a Norwegian military officer who led a puppet government after a pro-Nazi coup in 1940 when the Nazis invaded Norway. His name became a word for a traitorous collaborator.