He was just playing with Bum like he always does and it got out of hand. He was also a bit angry about everything that happened. Remember how Bum easily agreed for his jerk of a 'friend' to have the cake and Sangwoo was annoyed, he wants Bums to to have a backbone and not be so... Bum.
That's my take.
nope if u read it again you'd c he didn't agree to giving him the cake the guy just said that because he knows bum is anti-social and can't speak for himself
Because Bum didn't "stab" the uncle.
http://www.mangago.me/read-manga/killing_stalking/mf/c024/28/
Bum may have protested weakly earlier but he didn't do so in front of Sangwoo. So...
Still, I also don't think he agreed to give him the cake. If anything, Yoonjae was trying to manipulate the situation. He knew Bum wouldn't speak out because he knew Bum is meek.
any theory on why he was so pissed?