Samithi and sabha were 2 bodies of ministers and council which controlled the unlimited powers of the king. Prajapathi - as mentioned in the vedas - is but a personification of the governing body. Personification is a frequent occurence in sanskrit literature and it confuses a novice present day reader.
Time and again, kings rose to power during which period the council was subordinate since the king was all too powerful. Compare this with a very charismatic politician of today. When he seems invincible he does not bother too much about the council as he is confident that he has the support of people. Same case
Different kinds of yagnas were introduced to sanctify the king and establish powers. Rajasuya, Vaajapeya, Aswametha - all were characterized by animal sacrifices. Animal sacrifices in general are less pronounced versions of Human sacrifices. Even as far as Mahabharatha period, we find human sacrifices. Aravaan gives up his life so that pandavas can win the war.
As one reads the history of the world, the overall thought process of human has been more or less the same irrespective of geography - class and creed.
The kings of 3rd and 2nd milleneum in egypt were considered gods by their subjects. Even as far as 1 AD this continued - Julius ceaser statue was worshipped in greece temples.