From Gineipaedia, the Legend of Galactic Heroes wiki
Martin Otto's own father, Baron Siegmeister, had inherited both the barony and his title of Reichsritter from his own father. Yang Wen-li believed Siegmeister found meaning in his life by prosecuting commoners who sympathized with "unacceptable philosophies", like republican democracy, while working for the Department of Social Discipline within the Empire's Ministry of the Interior. He was said to be a dilegent worker, and often arrested far more people for "thought offenses" than his colleagues. The baron Siegmeister also obtained more confessions via torture than his peers, who were said to have been disconcerted by his tenacity and ruthlessness.
At home, Yang believed the baron to have been a "degenerate" man who beat his wife and son, Martin Otto von Siegmeister as well.
The baron brought home confiscated literature (about republican democracy, etc.) which he studied in order to better know his enemy. As a result, his home housed many 'forbidden books.' Ironically, Martin Otto was exposed to the ideas within those books, and learned to sympathize the ideas expressed inside them, opening his mind to the concepts of social justice and representative rule. As a young man, Yang believed Martin Otto von Siegmiester must have devoted himself to changing the unjust society of the Empire, but was cautious enough to never voice those opinions. He graduated from the military academy and earned several promotions. Slowly and deliberately he organized a a network of spies holding similar beliefs within the Empire's military.