With comics commonly featuring complicated timelines, elaborately crafted story arcs, and more characters than there are actors in the world, adapting them to a film format is always going to require some sacrifices in terms of content and tone. Nowhere is this more apparent than in a character's origin story, which is one of the most common parts of a hero or villain's arc to be rewritten in order for them to fit into the film universe.
Be it that this origin doesn't fit into the universe's current timeline, that it features characters that aren't yet introduced to the series, or just that it's too damn long, time and time again we see comic characters' backgrounds transformed into something almost totally different, with varying levels of success.
On the plus side, this does mean that fans who got into specific franchises due to their film adaptations have a pleasant surprise in store for them, as it means they can read through said origin stories without having heard it all before.
With comic films being even more popular than ever before, it's likely we'll be seeing no shortage of rewritten comic origins for the foreseeable future - for better or for worse.
11. Professor X - X-Men: First Class (2011)
The weird thing about X-Men: First Class and its origin stories is that, while Magneto's is kept entirely the same, Professor X has huge portions of his past changed for reasons that aren't immediately apparent.
While the fact the Xavier came from a wealthy family is still within the film, we learn that in this universe, the telepath met Mystique in his childhood, with the two bonding over their shared experiences as young mutants. This is especially interesting, as Charles would meet the blue mutant considerably later in the comic side of the franchise, and the two sharing no lifelong bond as they do in the film.
As is often the case with these types of changes, this was likely done to avoid having centuries-old characters within the film series, as where the film has Raven be a child in 1944, the comics have her as an already grown adult in 1921.