It's like anything else in mythology...there are at least four or five accounts of his death ...some say it's a scorpion sent by Gaea, for instance. According to Hyginus, Apollo objected to Atermis' affection for Orion, and when Orion was swimming, challenged her to hit that moving mark. She did, killing him, but she didn't MEAN to kill Orion, and in fact was tricked by Apollo into doing so.