The brother definitely saw his "hooligan" behaviour kinda encouraged by his dad, and also I think there was a time when he actually stood up for him (or maybe it was even the ML, I cannot remember exactly), but ...sure, there is the difference in temperament, like some kids are calm and peaceful and quiet (like me for example), others can bring hell and they're a ball of energy since day 1 (like my brother, I really have this real life example). And at least in the drama the brother definitely craved the attention and approval from his father and envied the closeness he saw between his dad and the ML, so yeah, the soup didn't start out too healthy either.
Besides, when your dad is living his life under the literal dog-eat-dog philosophy you adapt to the world as being this savage jungle where you resort to whatever means to survive... and they were both heavily influenced by that model, Ik it's a drama, but it's quite literal one became the man of the law and the other one was a law breaker, so yup...- one ran away from that image and the other took it to heart, the main purpose being to survive and protect themselves.