Adventure 330 (March 1965) begins a 2-part story by Jerry Siegel that sees Dynamo Boy join the Legion, and tear it apart.
Dynamo Boy is really Vorm, part of a space pirate brigade, who comes to Earth specifically to join and destroy the Legion. He impresses Star Boy with his powers, and gets a try-out where his radiation powers far outshine the other applicants.
Eye-ful Ethel, a reject, does return in the 80s, but is only used really well by Geoff Johns in his Legion story in Action Comics in the 2000s.
Once he becomes a member he sets out to have the rest of the Legionnaires expelled. He frames Colossal Boy and then Mon-El, prevents other Legionnaires from getting messages from the Science Police, appoints himself temporary leader and tosses them out.
By the end of the story he is the sole remaining member of the Legion, and sets out to build a new team, this time of villains.
The story concludes next issue.