why should we always treat all rational beings as ends and never merely as means
Kantian ethics Kant argued that rational beings can never be treated merely as means to ends; they must always also be treated as ends themselves, requiring that their own reasoned motives must be equally respected. This derives from Kant's claim that reason motivates morality: it demands that we respect reason as a motive in all beings, including other people. A rational being cannot rationally consent to being used merely as a means to an end, so they must always be treated as an end.[24] Kant justified this by arguing that moral obligation is a rational necessity: that which is rationally willed is morally right. Because all rational agents rationally will themselves to be an end and never merely a means, it is morally obligatory that they are treated as such.[25][26][27] This does not mean that we can never treat a human as a means to an end, but that when we do, we also treat him as an end in himself.[24]