Folklore all over the world says that people holding grudge for being killed unjustly will return as ghosts for revenge.
Examples (Wiktionary)
Mandarin
诊断通常是由拥有神秘力量和鬼魂交往的能力并有足够能力来战胜和驱走它们的人来作出，如巫医、术士、萨满、祓魔师。
診斷通常是由擁有神秘力量和鬼魂交往的能力並有足夠能力來戰勝和驅走它們的人來作出，如巫醫、術士、薩滿、祓魔師。
zhěn duàn tōng cháng shì yóu yōng yǒu shén mì lì liàng hé guǐ hún jiāo wǎng de néng lì bìng yǒu zú gòu néng lì lái zhàn shèng hé qū zǒu tā men de rén lái zuò chū ， rú wū yī ， shù shì ， sà mǎn ， fú mó shī ．
Diagnosis was usually done by people who have secret powers and the ability to communicate with ghosts, as well as sufficient ability to overcome and drive them out, such as witch doctors, sorcerers, shamans and exorcists.