Here's my opinion about "why the seme chooses to hurts his father's son over the father himself" is when someone hurts the one you dearly love, it'll make you suffer even more (that's my own speculation tho). I think the seme thinks that Qing Yu is one of the dearest person to his father, that's why he chooses to takes revenge to Qing Yu (to makes his father more sufferer)
Here's my opinion about "why the seme chooses to hurts his father's son over the father himself" is when someone hurts the one you dearly love, it'll make you suffer even more (that's my own speculation tho). I think the seme thinks that Qing Yu is one of the dearest person to his father, that's why he chooses to takes revenge to Qing Yu (to makes his father more sufferer)