This capacity to act freely,Kant tells us,is what gives human life its special dignity.Respecting human dignity means regarding persons not just as means but also as ends in themselves.And this is why it's wrong to use people for the sake of other people's well-being or happiness.This is the real reason,Kant says,that utilitirianism goes wrong.This is the reason it's important to respect the dignity of persons and to uphold their rights.
So,even if there are cases,remember John Stewart Mill said,"well,in the long run,if we uphold justice and respect the dignity of persons,we will maximize human happiness."
What would Kant's answer be to that?What would his answer be?Even if that were true,even if the calculus worked out that way,even if you shouldn't throw the Christian's to the lions because in the long run fear will spread,the overall utility will decline.The utilitarian would be upholding justice and right and respect for persons for the wrong reason,for a purely a contingent reason,for an instrumental reason.It would still be using people,even where the calculus works out for the best in the long run,it would still be using people as means rather than respecting them as ends in themselves.