Difference between sane people and insane people

What is different between sane people and insane people/abnormal people? An insane person can be seen as sane in this context of group but insane for the other.  Look at Hitler.  People would say what he did and planed how to kill all the Jews on earth.  That is insane.  But German people did not think it was so insane. They had good reasons to go alone with him to quite extent.  Many of them did anyway.  Jewish people had a very strong influence on all people on earth. 160422