Perhaps the berserkers were just guys that got off on combat, I have known a few people like that myself.  They are fairly hard to control but it is possible.  It does not take drugs to get the right type of person to go apes*** in combat and get into a killing frenzy.

Perhaps etymology can help ! Berserker : beer - seeker ... They were searching for a fresh beer after such an exhausting voyage and got mad when they realised beers were warm in Anglia !   Just a hypothesis ...  :-

Maybe thats why we were so angry in Iraq? ;D