Hmm, this sounds like a variation on the argument I once heard about would Logan's adamantium sheathed claws stop a lightsaber blade. I guess one of the difficulties is they are from different universes (no adamantium exists in SW verse and no SW lightsabers exist in the Marvel verse), so ultimately we can never know. However in Wolverine Weapon X: Adamantium Men storyline involved a bunch of assassins turned into Wolvie copies, with a nano-based healing factor, enchanced senses and abilities, adamantium bones, and most importantly for this, laser claws. They were unable to cut through Logan's bones (just as his claws couldn't cut through them). So I guess a lightaber equivalent in the Marvelverse (some form of laser/energy blade) couldn't go through adamantium (based on that story anyhow).
If there was ever a lightaber that could, well then it could decapitate him, which might kill him.
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