It is worthwhile to note that Professor Rummel of the University of Hawaii puts the democide deaths during the Cultural Revolution at around 7,731,000. Democide of course being mass murder by government.
Details here:
http://hawaii.edu/powerkills/NOTE2.HTM
Once totalitarians have power they no longer have use for revolutionaries. In fact, the revolutionaries that bring them to power become a threat and need to be eliminated.
Brownshirts were among those targeted on the night of the long knives in Germany. Che Guevera fled Cuba after helping Castro gain power because he knew that Castro would need to kill him.