|Frankenstein (1968) [Dr. Frankenstein/Frankenstein's Monster]: As Dr. Frankenstein, he is presumably killed by his creation (also played by Ian). (I haven't seen this movie myself, so I don't know for certain whether either Frankenstein or the monster, or both, die in this version.)
Alien (1979) [Science Officer Ash]: Playing an android, he is decapitated with a fire extinguisher by Yaphet Kotto while Ian is trying to kill Sigourney Weaver; his severed head is then re-activated by the other crew members, and is finally destroyed with a flamethrower by Yaphet. (Thanks to James and Philip)
All Quiet on the Western Front (1979) [Himmelstoss]: Presumably shot to death by French troops on the battlefield. (I haven't seen this version, but I have seen John Wray's death in the 1930 version.)
Inside the Third Reich (1982) [Josef Goebbels]: Shot to death (along with Elke Sommer) by a German soldier, on Ian's own orders. (I haven't seen this, but its timeframe does cover the postwar period after Goebbels' death.) (Thanks to PortsGuy)
Dreamchild (1985) [Reverend Charles L. Dodgson a.k.a. Lewis Carroll]: Dies (off-screen) of pneumonia, in between the 19th-century flashback sequences and the 1932 "present-day" scenes.
Hamlet (1990) [Polonius]: Stabbed to death through a curtain by Mel Gibson, who had mistaken him for Alan Bates.
King Lear (1997) [Lear]: Dies of old age, compounded by heartbreak over the death of his daughter (Victoria Hamilton). (I haven't seen this version, but I'm familiar with the story.)
eXistenZ (1999) [Kiri Vinokur]: Machine-gunned across the torso by Jennifer Jason Leigh outdoors at night. (This later turns out to be part of a virtual-reality game, and not an actual death.)
The Day After Tomorrow (2004) [Terry Rapson]: Frozen to death (off-screen) due to the massive climactic changes; we last see him when he tells Dennis Quaid that it's too late for Dennis (or anyone else) to save him. (Thanks to Mathew)
Lord of War (2005) [Simeon Weisz]: Shot in the head by Nicolas Cage, when Nicolas is forced to shoot him. (Thanks to Gary)