The berserkers were a subset of Viking warriors who went into battle wearing wolf or bear skins instead of armor and fought with uncontrollable fury, which is where we get the phrase "going berserk." While in their battle state, they were a danger to even their own compatriots. The berserker fighting style has been attributed to a self-induced trance, drugs, or possibly mental illness. Life couldn’t have been easy for a berserker, especially when they had time to confront moral dilemmas. This comic is from Zach Weinersmith at Saturday Morning Breakfast Cereal.
https://en.wikipedia.org/wiki/Cú_Chulainn