It really is no secret many Marvel Cinematic Universe fans disliked Avengers: Age of Ultron from 2014.

One thing for sure is the use of Pietro Maximoff (aka Quicksilver) seemed like a wasted opportunity in a few eyes.

However, he eventually became a key Avengers ally, particularly in attempting to bring down Ultron.

Despite siding with Ultron at first to bring down Tony Stark, Pietro turned to The Avengers side when realizing what Ultron wanted to do in destroying all humans.

Why this occurred was likely due to Pietro not having enough character development, if also not gaining enough audience sympathy.

Having Ultron similar to Stark made him akin to an evil human brother rather than just a robotic figure.