In war, these bullets are banned by the Geneva convention. Wounds are hoped to be survivable by humans and the bullets are intended to poke a hole in enemy bodies that removes them from battle.
And in terms even a psychopath would understand:
A dead soldier takes one soldier out of action. A wounded solder takes two, plus a medic's time and a drain on the supply lines to provide food, medical attention, and transportation away from the front.
Fewer dead soldiers and wounds with better recovery means fewer vendettas and broken families. This makes it easier to make peace and interact peacefully with a former enemy in the years after the war, and for the warring parties' economies to recover once peace breaks out.
Expanding bullets, on the other hand, are the ammo of choice for personal protection - whether civilian or police. They are more likely to incapacitate an attacker (when fired, as is typical, from a moderate-powered handgun rather than a rifle) and less likely to penetrate an attacker or wall and continue on to injure an innocent bystander. (Generally, deadly force is only justifiable in self-defense until the attack is stopped. Even with expanding bullets it's only about one in five that a person shot until incapacitated actually dies.)