In which movie did Magneto kill the president as stated in X-Men: Days of Future Past? I have been trying to figure out why they had him in prison during the events of DOFP.
As far as I'm aware this was never shown as part of a film but was actually used in the trailer for X-Men: Days of Future Past and the promotional material for it. The trailer that first showed this is below:
This then went on to use a website with a heck load of material providing lore into what happened to get people excited for the film. However, the site has since gone down but per some of our users efforts most/all of it has been recovered in this related question.