Worst is he did it before.

If you trusted (voted or vouched) for someone to be part of a team, say football. Then during the match day, he turns and joins the other team. Or if it was a war between A and B.

Define traitor: a person who betrays someone or something, such as a friend, cause, or principle.
"he was a traitor to his own class"
synonyms: betrayer, back-stabber, double-crosser, double-dealer,