Die? Don't you know? Spartans never die.
Darth Vader will go back in time and kill him in halo 6
He will die in the least heroic way possible, such as just walking down a sidewalk and getting sniped in the head by some psycopathic dude with a BB gun. The BB will puncture his armored helmet and his skull and kill him.
Just a note, Darth Vader wouldn't go BACK in time, as he is from a galaxy a LONG, LONG TIME AGO in a galaxy far far away. Your argument is invalid.