Ikr? Everything makes sense once you realize it's his brother. That fucking nasty attitude xD
What? No, he was pissed off at her for the kid getting Bum hurt and he thought of the kid as a younger version of himself causing his mom problems, not his little brother.
Obviously it wasn't his little brother, goodness me. Sangwoo was just triggered by the mother and child, and projected his own mother's image onto her. At the end of the day Sangwoo is not a mentally well man, and he is experiencing increased distress related to his mother, as evidenced in that scene.
This^
Ohhh So That Annoying Brat Is Sangwoo's Little Brother....It Seems Sangwoo Has Lost It Bum Should Protect His Pathetic Bum...After Thinking, He Got Trolled By A Kid *Sigh* Hopeless -_-