Even before joining the team, Charlie has a looong history of betraying whatever team he's on to their sworn enemies, whatever side of the good/evil divide they're on.
Downplayed: Charlie wasn't a traitor to start, he simply deserts instead of defecting after a morally questionable action. He doesn't help them but he doesn't aid their foes either beyond his leaving weakening them.
Originally may have been that instead of Charlie defecting to the Five-Bad Band, it was Tony who defected to the Five-Man Band instead. However, both forms are now so common the inversion is rarely recognized as such.
Every member of the team save Tony or Charlie defects to the opposite side, leaving only them.
Enforced: Creators want somebody to be a traitor but do not want anybody who audience already knows and likes to be him.
Lampshaded: "We should have suspected that if you had not been with us from the beginning you could betray us."
Defied: The heroes double check everybody who wants to join them knowing that he may be The Mole.
Deconstructed: Even if he truly wanted to, Charlie, having defected to the Five-Man Band, finds that he is trusted by absolutely no one on the team. In fact, they distrust him so heartily, that routinely give him the most dangerous assignments not to trust him with their success but to occasion his demise.
Reconstructed: The Five-Man Band realize that they can't function without trust, even if the only one they distrust is Charlie it would sour relations between the original five eventually. So they engineer a series of secret tests to find out whose side Charlie is on, and so that if he really is evil the intelligence he takes back helps them take down the Five-Bad Band.
Played For Laughs: Charlie is so comically obvious as a traitor that the rest of the Five-Man Band have to fight to keep a straight face while feeding him false intel.
Played for Drama: Charlie is genuinely sad about betraying the Five-Man Band, and realizes that afterwards no one will trust him, and his evil associates will probably dispose of him once he's no longer useful.