berserk

(noun) one of the ancient Norse warriors legendary for working themselves into a frenzy before a battle and fighting with reckless savagery and insane fury. Synonyms: berserker.

(adj) frenzied as if possessed by a demon. Synonyms: amuck, amok, demoniac, demoniacal, possessed(predicate). Examples: "The soldier was completely amuck." "Berserk with grief." "A berserk worker smashing windows."