And also there is sometimes where a good guy decided to get a bad guy image, more precisely a delinquent image, because maybe good is boring. Especially the case of Duncan in Total Drama Island unless he's a bad guy with a gold heart.
In Goldorak (UFO Robot Grendizer), in episode 7, Janus(Gorman), a member of the Vegan bodyguard (known in French as "Division Ruine") was the 1st serious opponent of Actarus(Duke Fleed)
http://www.maxtoons.com/Gorman.htm
But Gorman didn't check his "ally", General Hydargos (Blackie) who didn't like the idea of a promising young captain defeating Actarus and Janus was betrayed by Hydargos allowing Actarus to win the fight.
episode 29. Commandant Atlas (Haruk) was a good guy forced of being a bad guy, he had joinned the Vegan troups because his planet was an ally of the Vegan empire and was disguised as an young Earthmen.
When Mizar (Goro) discovered his secret, Atlas wanted to eleminate him but after a long hesitation, decided to let Mizar go.
Another "special bad guy guest star", was Ergastul (Gauss/Goss) in episode 34
http://www.maxtoons.com/Goss.htm

Gauss decided to let go Actarus for a duel at the night of the full moon. In the meantime he fight against Actarus (Duke Fleed) in person but fall over a cliff in a river and was rescued by Actarus. He decided to throw the final fight