Verb
go postal (third-person singular simple present goes postal, present participle going postal, simple past went postal, past participle gone postal)
(intransitive, chiefly US, informal) To become aggressive and erratic, especially due to stress; specifically, to carry out a shooting spree at a workplace environment; also (more generally) to become very angry; to lose one's temper.
(to become aggressive and erratic): Synonym: run amok
(to become very angry): Synonyms: see Thesaurus:lose one's temper