This was back in Avengers v.1 no.82 1970. Aries, first leader of Zodiac, was threatening to destroy Manhattan from his airship. Thor destroyed the airship and presumably everyone on it with lightning. I understand the urgency of stopping Aries, but Thor, like all Asgardians, was pledged never to kill a mortal, or face the consequences from Odin (at least in those days). Now maybe this was magic lightning that sent Aries to another dimension for a time, but all references say Aries is dead as a doornail. Maybe this is why Thor became unworthy then.