In the Marvel Cinematic Universe, Thanos was killed on two separate occasions:
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Initially, Thor killed Thanos. In Avengers: Endgame, after discovering Thanos had destroyed the Infinity Stones, a grief-stricken Thor beheaded him.
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Ultimately, Iron Man (Tony Stark) killed Thanos and his army. Later in Avengers: Endgame, during the final battle, Iron Man used the Infinity Stones to snap Thanos and his entire army out of existence, sacrificing himself in the process. This is generally considered the definitive death of Thanos in the MCU.
