Thor had the power to end it instantly, but he wanted Thanos to know who was standing in front of him. After losing Asgard, Heimdall and Loki, that moment was personal. The chest shot wasn't a lack of power, it was Thor choosing revenge over efficiency, and Thanos got the last laugh.
Was Thor’s decision to aim for the chest instead of the head pure ego… or did he really just want Thanos to see who was killing him?