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.
Or, Tony knows that it would be worth it to fight against the Five-Bad Band to uphold justice, even if it costs his life.
Parodied: When Charlie joins most of the team starts taking bets as to whether or not he'll turn within two years.
Zig Zagged: ???
Averted: The Five-Man Band suspects that Charlie may really be Tony, The Mole, only to find that, no, despite their distrust, he really is the good guy.
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.