Note to Twelfth Night, 5.1.118, "Why should I not (had I the heart to do it) / Like to the Egyptian thief at point of death, / Kill what I love? (a savage jealousy / That sometimes savours nobly)"

The "Egyptian thief" is generally believed to be Thyamis, a character in An Æthiopian Historie (often referred to as "Ethiopica") by Heliodorus. Thyamis, the brave and noble leader of a band of thieves, has taken from another band of thieves the captive Cariclia, a beautiful Greek maiden. Thyamis falls in love with Cariclia and wants to marry her. She consents (falsely, unknown to Thyamis), but immediately aftewards Thyamis' band of thieves is attacked by a larger band. Thyamis hides Cariclia in a cave to protect her, but comes to believe that he will lose the battle and so he kills her. (Actually, he unknowingly kills another woman, not Cariclia.)

The Meeting of Theagenes and Chariclea by Ambroise Dubois

The tale was popular and Shakespeare could have just picked it up, but if he actually read An Æthiopian Historie and had a particular passage in mind, perhaps it was this one:

[Thyamis] in haste went to the Cave. Surely a barbarous nature cannot easily be withdrawen, or turned from that, that he hath once determined. And if the barbarous people be once in despaire of their owne safetie, they have a custome to kill all those by whome they set much, and whose companie they desire after death, or els would keepe them from the violence and wrong of their enemies. For that same cause also Thyamis, forgetting all that hee had to do, being encloased with his enemies armie, as if he had been caught in a net, almost inraged with love, jelousie, and anger, after he came in haste to the cave, going into the same, crying with a lowde voyce, and speaking many thinges in the Egyptian tongue, as soone as hee hearde one speake Greeke to him about the entrie of the cave, and was conducted to her by her voyce, hee layde his left hande uppon her head, and with his sworde thrust her through the body, a little beneath the paps. And after this sorrowful sort, that woman giving up her last, and gastly groane, was slaine. But he, after he came out, and had shut the doore, and cast a little gravel thereon, with teares, said, These espousals hast thou at my hand . . . .¹

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     ¹Heliodorus, An Æthiopian Historie, trans. Thomas Underdowne, The Tudor Translations 5 (1587; 1895; New York: AMS Press, 1967) 38-39.