Is it only a recent trend (the last three years) that comic book movie adaptions might not actually show the villain actually dying? Maybe they only show him disappearing into something or the thing he was in explode or whatever. It seems to me it would make more sense to end the movie that way so that if you ever wanted to you could try to bring the character back.
If you made things too graphic with the way how the bad guy died, it would make it more implausible bringing them back (if you ever wanted to). I would think you would most hate to more-obviously kill off the heroes main nemesis. If you would wish to bring back any villain I would think it would have to be your nemesis.
Are there any older comic book movies out there where they seem to have made the villain's death deliberately vague?
In regular action movies usually the death of the main villain is really obvious. You see them get squashed by a two ton weight or something.
