We’re not surprised that being born a mutant is a shortcut to superhero status. But I read mostly DC comics when I was young, and am not as familiar with the Marvel pantheon as I could have been, so I was surprised at how many characters they used as both X-Men and Avengers. Some superheroes just get around! And even more surprising was how characters can go from evil to hero (and back again). Take Quicksilver, for example.
In Avengers #16, Stan Lee and Jack Kirby did, at the time, the unheard of. The legendary fathers of the Marvel Age replaced the current roster of Avengers with an entirely new team, mostly comprised of former villains. Joining Captain America on the team were Scarlet Witch, Hawkeye, and mutant speedster, Quicksilver.
Before he joined the Avengers, Quicksilver was a member of the Magneto’s Brotherhood of Evil Mutants. He was a reluctant villain whose main motivation was protecting his sister from Magneto’s machinations. When he and his sister joined Cap on the Avengers, a cohesive history between the X-Men and the Avengers began.
And he’s not the only one. Read the backstories of 14 characters who were both X-Men and Avengers at on time or another, at Den of Geek.