The Feeblemind spell, at first glance, looks like a brain-blasting spell. In the details of the description, though, it talks about the breaking of the mind and personality, but the creature does not lose the will to fight.
Does it mean a calm person would be turned into a madman or berserker by necessity? Or does it require the DM’s discretion?
On a failed save, the creature's Intelligence and Charisma scores become 1. The creature can't cast spells, activate magic items, understand language, or communicate in any intelligible way. The creature can, however, identify its friends, follow them, and even protect them.
The rules do not specify the target creature's exact behavior and assuming the target is NPC controlled by the DM they will have the final word in what the creature does.
That being said feeblemind lowers target's intelligence score to 1 which is comparable to many creatures, commonly beasts in the Monster Manual. Due to this, I would say it is reasonable for the target to act similarly to how an animal or a beast would act in any given scenario, including protecting its friends (as mentioned by the spell) and attacking, or running from enemies it perceives as a threat.
