It is all about the pursuit of Power. The Nazis murdered other socialists who were not Nazis. If they had been members of the National Socialist German Workers’ Party they would have been fine. Competing brands of Socialism always seek the anihilation of their competition.
“We are socialists, we are enemies of today’s capitalistic economic system for the exploitation of the economically weak, with its unfair salaries, with its unseemly evaluation of a human being according to wealth and property instead of responsibility and performance, and we are determined to destroy this system under all conditions.”