The symptoms stem from the accumulation of a deadly neurotoxin that results most often from a lack of violence or old age (as any self-respecting warrior minifig will die before their midlife crisis). The toxin causes the minifig to become more serene, kind, and generally unbearable until the primal instincts break out. The minifig loses control of their body, and commits suicide. This suicidal instinct is thought to have evolved in order to ensure the disease does not spread, to prevent the minifig from dying of (SHAME!) old age, and to reverse the loss of violence caused by the disease.
Very few cures are known, one of which being major changes in lifestyle that shakes the minifig (see Dr. Fireo), or being dropped into an especially gory battle.
Superstitious tales are also told of a Negaverse entity called The Peacenik. In these tales it is said that if you don't die in battle as a good minifig should, then the Peacenik will come for you and take you away to a land of peace and there will be Folk Music and Humus.