In a least convenient possible world, where the serial killer really enjoys killing people and only kills people who have no friends and family and won’t be missed and are quite depressed, would it ever be conceivable that utilitarianism would imply indifference to the choice?
Also, would the verdict on this question change if the people he killed had attempted but failed at suicide, or wanted to suicide but lacked the willpower to?
Also, would the verdict on this question change if the people he killed had attempted but failed at suicide, or wanted to suicide but lacked the willpower to?