It’s Crazy how some reacted to Byul talking back to Hyesung as if they never experienced something like this as a teenager. Of course it is not good to do things behind his mom’s back or talking/shouting back but putting it in Byul’s perspective, if you were confronted like that top with guilt of what you did, like a cat drove into a corner you will also spat back, not necessarily because you are disrespectful but because you have no chance to process what was happening.. Byul is not mature enough to asses things calmly lol.. and besides as an adult shouldn’t have Hyesung and Dojin explained things to Byul clearly.. And dragging Byul into the rift between Hyesung and his mom is just not right.. not when he haven’t heard it clearly from Hyesung. 
In the end Byul is still a good and considerate kid.. seeing how instead of blaming his parents he cried, felt guilty, and got worried of how he hurt his mom.. So stop with the “Hyesung should have had abortion” that’s crazy.. if Hyesung can read that He’ll beat your ass hard.