首楞嚴義疏注經 8.468

8.468

即前怨習為因也。由忿為先，懷惡不捨，結怨在意，熱惱居懷，受餘報時亦假毒類，即蛇虺、毒虫有靈者，成蠱毒鬼。

This is caused by the previously mentioned habit of animosity. Preceded by wrath, harboring malice without letting go, harboring resentment within the mind, and nursing torment within the heart—when experiencing the residual retribution, one also borrows venomous types. That is, venomous snakes, vipers, and poisonous insects that possess spirit become poison-sorcery ghosts.