The unstoppable juggernaut that is the Marvel Cinematic Universe (not to be confused with the Juggernaut that Fox owns) is easily the most popular film franchise in the world. Each blockbuster epic has been seen by millions of people and are equally loved by fans who dig seeing their favourite characters on the big screen and newcomers who have never picked up a comic in their lives.
A big part of the MCU's success is down to the decades worth of great stories that the films have to pick from and the mixing and matching of various characters and storylines to condense hundreds of issues down into a single narrative.
A side-effect of this though is that a lot of characters and events go through changes during the transition from print to the big screen, either to bring them more in-line with modern standards or to help them better fit in with the overall narrative. Some of these changes have been pretty minor like ditching Thor's helmet or changing Heimdall's race (although some people would have you believe that's an almighty injustice, inevitably).
Sometimes though, these changes are so huge that a hero or villain can seem like a completely different character.