In the aftermath of Avengers: Infinity War’s theatrical release, many fans were blaming Star-Lord (Chris Pratt), whose actions interfered with the Avengers trying to remove the Infinity Gauntlet, for Thanos’s survival and victory but blame could be laid at another hero’s feet, Thor (Chris Hemsworth).
Shortly after Thanos obtains all six Infinity Stones, Thor emerges from the heavens with his powerful axe Stormbreaker, which strikes Thanos in the chest, not the head, a decision that ultimately resulted in half of all life being snapped out of existence. Writer Christopher Markus revealed on the Avengers: Infinity War commentary track that Thor’s decision was fueled by revenge, while adding a humorous anecdote about when this scene was shot.
Source: IGN.com Infinity War: Why Thor Didn’t Take the Headshot