any theory on why he was so pissed?
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.
http://www.mangago.zone/read-manga/killing_stalking/mf/c024/28/
Bum may have protested weakly earlier but he didn't do so in front of Sangwoo. So...
Connor and simon invented true LOVE.