The repartee
"Don't unnecessarily make me your enemy" This statement from Cassian felt more like a threat than a request to Helio. Both of them were family but didn't have affection for each other or that was what Helio thought because Cassian on the other hand, respected Helio as a man and his uncle more so than he did his own father.
Cassian's father is a coward and he had come to terms with that. The emperor was so cowardly and weak that he failed to protect the woman he loved. Unfortunately, that said woman was Cassian's mother. His father let the empress hurt his mother as he was busy maintaining his duty and dignity on the other side.
According to Cassian, life did not have any meaning without our loved ones. Wealth and status.....meant nothing if we lost the one person whom we cared for. If sacrificing the one he loved was what was best then he did not want that. He rather be the worst of himself than lose that person.