Nam-gyu is shown to be a more violent, cruel and sadistic than even Thanos is. While Thanos treats his teammates with respect and is even upset by Gyeong-su's death, Nam-gyu bullies both Se-mi and Min-su frequently, to the point where even Thanos calls him out for it. But he is clingy, and attached. It is highly possible he experiences psychosis and BPD. His anxious disorders leading to drug addiction to feel normal or a sense of stability - because at least they can't leave him, too.