I vote noone. The benefit of killing any major characters to your current film will be outweighed by losing that character for the future of the Marvel Universe films. Unless you want to bring someone back from the dead and get the biggest eyeroll possible. Killing superheroes/titular characters is not clever any more (if it ever was). Now if you want to kill secondary characters that's not a problem.
Alternatively, write well and you won't need to depend on cheap tricks to create good drama!