i -think- what happened was that the big brother loved the lil brother but was constantly haunted by that and didnt want the little brother to stray from the right path, ie, become tainted, because he was the sun. so he punched the little brother to show everyone that the big brother has nothing to do with him. He sees himself as a useless, dirty thing and wants to devote his life to protect his little brother. But the little brother is already not "pure"; that's why he is pretending to be pure. If the big brother knew that the little brother had already become abnormal, he would just blame/hate himself more. So to prevent that, the lil brother pretends, and also "punishes" the big brother, which he thinks he deserves.
WAIT CAN SOMEONE EXPLAIN THE ENDING TO ME LIKE WHY WAS THE UKE MAD AND WHY DID HE GET TIED UP WHY DID THE SEME SAY HE WAS PRETENDING TO BE IN THE SUNLIGHT HUH IM SO CONFUSED BUT THAT SHIT WAS SEXC SO IM NOT COMPLAINING