The word berserk comes from two Danish brothers named Berserk who lived on an island off the coast of Denmark back in the ~1600’s or maybe even later. They lived there for some time, murdering everyone who visited the island in the most horrible ways and maybe even eating them to boot. This went on for a while, and at some point, a group of people undertook an expedition to the island and killed the brothers.
The brothers’ name became a word, Berserker, in the Danish language for anyone or anything that seemed out of control and maniacal. At some point, it got borrowed into English as berserk with the same meaning.