Average human welfare can be improved by eliminating everyone who is below average.
Except that if they still believe their lives are worth living, then you are causing them disutility by violating their preference to survive. It also causes everyone everyone else disutility because they don’t want other people killed, and because they become worried about themselves or their families dying if they become unhappy. It also eliminates the future possibility of the killed people’s lives improving.
Except that if they still believe their lives are worth living, then you are causing them disutility by violating their preference to survive. It also causes everyone everyone else disutility because they don’t want other people killed, and because they become worried about themselves or their families dying if they become unhappy. It also eliminates the future possibility of the killed people’s lives improving.