We are in a medieval fantasy world. There is a folk where, sometimes, a berserker is born. They are quite rare (maybe 2-5%) and there is no difference between a normal man and a berserker before he turns berserk.
I call berserker a man (or a woman) who will become some kind of a beast during a battle, and sometimes outside the battle. Anger and hate push the berserker thing, but the more the berserker uses its power, the more it occurs without consent. In this state, he can kill dozens of men, both friends and ennemies, before calming down and waking up exhausted.
During this time, he's invulnerable (no fear, very quick, very strong, can't feel pain, possibly can't be hurt at all - it's a magical power, not just a kind of madness)
There is nothing able to calm him down, just time (variable time). The end of battle often ends the berserker crisis, but not always.
It's a warrior folk. They often have fights with an army vs another army for territorial issues. They are quite civilised, and they don't want to kill outside war if it's not an absolute necessity.
Berserkers are very good at fighting, but very dangerous. One of them can defeat a big group of ennemy soldiers alone.
But unfortunately, if you steal his breakfast, your lifelong friend Mike could just kill you, and your friends, and your dog, and the village chief, and the old Martha passing by. Then calm down and cry.
What's the best way to deal with berserkers ?
How would a medieval/fantasy society will likely deal with them ? (That might be a different answer.)