Responses

Because it also gives Johoon some revenge on Jiwook for ripping that picture. Siwon (Taemin) already knew Johoon’s involvement so it’s not like that changes anything for Johoon, who doesn’t care what people think about him anyways. And really, Jiwook gets away with it too. There is only threat of blackmail, but Taemin throws away the recording so his only punishment is being confronted with what he did.
Why did Johoon helped Woojin at the end to reveal Jiwook's involvement in Siwon scandal well in fact Johoon is also part of it. Though Woojin is really at fault in the things that he has done to his twin brother, at the end the punishment was all in him, like Johoon and his gang got away from it and all blame is to Woojin. In the first place he don't feel resentment towards his brother(he's brother achievements are also like his achievement he said) not until people around them keep comparing them. When he tried to rebel he was involved to wrong people which he doesn't want to be further involved with, but Johoon took interest in him and lured him more in them.