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Aphrodite, Eros' mother, because she was jealous of the beautiful Psyche, asked Eros to shoot her with an arrow so that she would fall in love with someone repulsive at first sight, but Eros ended up falling in Love at First Sight with Psyche. And then Psyche's sisters persuaded her to peek at Cupid because they were jealous of her gorgeous home.
Hera, Zeus' wife, was jealous of all of his numerous conquests. Hence why anyone that slept with or had any romantic contact with Zeus (including the ones he raped) would suffer horrible deaths at her hand. It doesn't exactly help that she's the Goddess of Marriage and Family, as well. It also doesn't help that he would shag virtually anything with legs.
Cain couldn't stand the fact that his brother Abel's sacrifices were accepted by God and his weren't, leading him to commit the first murder. Since God accepted the blood offering his brother gave of a new lamb and rejected his crops (and hadn't got around to saying murder was bad yet), Cain may have just been confused. The story doesn't really give Cain or God clear reasons for their actions. One common interpretation is that Cain gave grudgingly while Abel gave willingly.
According to Jewish tradition, Cain had every intention to kill his brother, and since he didnít know how he just attacked all of him until he came across a spot that could kill him.
Many Bible stories have this as a motivator. Most of Joseph's brothers are jealous over their father's favoring of him, leading them to sell him; David wants Uriah's wife, so he sends Uriah to the front lines where he's likely to be killed in battle; Saul realizes that David will be king instead of his own son, and tries to kill him despite his loyalty to Saul. About the only one who can be rightly jealous is God Himself, probably because He knows what's going down with what and who belongs to Him; it's not just fearful speculation.