I was just reading some days ago how Buddhist emperor Ashoka the Great abolished discriminating policy of a Brahmin never being punished to death irrespective of the nature and severity of the crime while it was allowed for other castes.
Buddhism is one of the rare religion that does not recognize divine rights of kings. In Buddhist mythology a king is like everyone else who merely gets 1/6th of the harvest and maintains peace and protects from external threats. He is jist another man. Christinaity and Islam, on the other hand, do vastly believe in divine rights of kings.
The aspect of Hinduism that you mention was actually practiced, but the inner core of Philosophy tells the division among men emanate from their quality and practice. Birth does not matter. But, in mythology, when a low caste man named Shambuka tried to study the Vedas, the Ramayana epic king and Vishnu incarnation Rama murders him.