The ethical reasons behind someone's beliefs are different for everyone, especially on matters that tug on the heartstrings such as animal testing.  I have tried to compile several ethical beliefs from both sides to show a variety of opinions. 

Anti-Vivisection/ Animal Rights

         Many anti-testing supporters believe that we as humans do not have the right to harm non-consenting animals just because we can, in other words "might does not make right".   They believe animals have or deserve rights, which would include the right to live and not be exploited. Similarly, the innocent state of the animals used for testing is also a ethical reason behind many peoples belief's.  They are in the mindset that innocent things should not be inflicted pain upon for gain. 
Animal rights extremists such as PETA would even go as far as saying that they "feel that animals have the same rights as a retarded human child because they are equal mentally in terms of dependence on others."  or that we all deserve the same rights:"There’s no rational basis for saying that a human being has special rights. A rat is a pig is a dog is a boy. They’re all animals."
          Many people who are anti-cosmetic testing are pro-testing for medical purposes.  This is probably because they believe cosmetic testing is morally wrong since the deaths of animals are not used to preserve the lives of others.  More simply said, we don't 'need' cosmetics and therefore shouldn't kill to create luxury items. 
Famous anti-vivisection supporter Mark Twain's ethical justification.
An advertisement displaying anti-test beliefs that an animal life equals a human life

Pro-Medical Animal Research

        A common misconception about people who are for animal research is that they are for the killing of animals because they are against replacing animal experiment with alternatives.  In reality, pro-test people also are working to develop "cruelty-free alternatives" to animal testing.  Despite these efforts, at this time no alternatives are as accurate or useful as animal testing.  Knowing this, anti-testing supporters still accuse scientists of killing animals pointlessly when there are "alternatives".   Pro-test supporters understand that these so called "alternatives" are more complimentary methods than anything else and should be used alongside animal experiments to better results. 
        The basic ethical reason behind someones decision to support animal research is that they believe animal lives are not the same as human lives and therefore animal do not deserve the same rights as humans.   They may also believe that the killing of few now is justified by the saving of many later.   Many people support the experimentation of animals because of personal experience i.e. a relatives life saved via a product developed through animal testing.

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