Marvel's Avengers have featured numerous rosters and teams that have seen heroes (and sometimes villains) band together as Earth's mightiest heroes. However, one of the oddest inductees to the team was when the Spider-Man villain Sandman was offered a spot alongside Captain America, Thor, and Spidey.
Sandman has been a pain in Spider-Man's side since he first debuted back in The Amazing Spider-Man #4 by Stan Lee and Steve Ditko. After a freak accident, William Baker (also known as Flint Marko) is transformed into a sand monster, who has the ability to shapeshift his body due to the sand bonding with him. Sandman started as a supervillain and later a member of the Sinister Six, while occasionally becoming a good guy and fighting alongside Spider-Man. In one comic, his change of heart was rewarded with being named one of the Avengers.
In Avengers #329 by Larry Hama, Paul Ryan, Tom Palmer, Christie Schelle, and Bill Oakley, the Avengers gather together to introduce a new team consisting of Thor, Sersi, Vision, Quasar, Black Widow, She-Hulk, and Captain America. After the new Avengers team is officially assembled, Captain America reveals he's adding seven reserve substitutes to the team. Five substitutes are familiar faces: Spider-Man, Black Knight, Falcon, Hercules, and Captain Marvel. However, when Captain America makes the announcements he reveals two more additional substitutes: Rage and Sandman.
The media immediately presses Captain America about his decision to allow the former criminal to join the Avengers. Cap tells them that Sandman has been completely rehabilitated and granted a Presidential Pardon. While it might seem strange to make one of Spider-Man's biggest villains an Avengers, at that point in his comic history, he had assisted Spider-Man and the Avengers as part of the Outlaws. Being named a reserve member of the Avengers was his next step towards becoming a full-fledged good guy. Villains have turned into Avengers before.
Sandman would eventually return to his evil roots, but for a decent amount of time, the villain was a good guy and helped the Avengers when needed. In the time since joining the Avengers as a substitute member, Sandman has flipped back and forth of being a hero and a villain. No matter which side he was on, the fact he went from a serious, A-level Spider-Man villain, to a substitute for the Avengers, shows that sometimes even bad guys, such as Sandman, can turn things around.