The very nature of comic book movies makes satisfactory endings difficult to come by. These days, very few exist outside of a long-running series of shared cinematic universe, which leaves little room for ambiguity, indefinite closure or the (permanent) death of a major character in the final act.
A botched conclusion has the potential to undo the two hours of solid gold which came before, leave the punters clamouring for refunds at the ticket desk and even kick a reboot into production to undo the offending events.
X-Men: The Last Stand is a solid example of the latter scenario, a movie which charged to the finish line too quickly, hit fan-favourite characters with meaningless deaths and rendered its big emotional moment pointless with a post-credits scene. Although the rest of the film wasn't much to write home about, the finale was so heinous that X-Men: Days of Future Past had to obliterate it from continuity.
Despite the challenges filmmakers face in the superhero space, there are many examples of comic book movies that brought the curtain down in spectacular fashion, usually when the director wasn't obliged to set up a dozen future sequels and spinoffs, and these are moments the fans will never forget...