Bum doesn't want to run from Sangwoo anymore.
He tried. The first attempt failed miserably and he get beaten up. Then he tried again at school festival and also failed. Bum isn't really smart, he could scream at the mall but his social anxiety would make it harder. Also Bum is secretly enjoying his companionship with Sangwoo except the torture.
And also, he's got a bit of Stockholm Syndrome mixed with fear, and no real person he trusts... And now he feels he can't trust the police either because of helping murder people. It makes sense he is no longer escaping. Or even trying to get word out.
Bum was initially a stalker, he was obsessed with Sangwoo before he broke into the house. He was afraid of the torture, but Bum's obsession towards Sangwoo doesn't fade. No Stockholm Syndrome.
A lot of the times people who are kidnapped don't run because they're afraid of the consequences if they're caught. Ariel Castro would purposely leave the door open occasionally, and when one of the women would try to escape he'd beat her. He conditioned her into giving up escape. She was actually hesitant to escape when it wasn't a trick because of the conditioning. Luckily she overcame that fear and escaped. Point being that it's not always as easy as running away.
Why didn't Yoonbum just run away from the store?? Like dude, u can flipping get free from Sangwo and everything!! WtfΣ( ￣□￣||)