No way... So, that cop trick Yoon Bum to make him confess everything. Like that?
Yes. The cops separate two suspects, then get them to believe the other has or will confess, which pressures the other suspect to confess first. The two cops know Sangwoo won't tell them anything, so they're trying to get something out of Bum. That was the "trick" Yang mentions.
possible, but we don't know for sure since we only seen Bum's point of view.
Sangwoo, as a sociopath/psychopath would not out himself. Watch Hannibal: you'll get the idea. If Sangwoo ratted on Bum, he would be outing himself as an accessory. PLUS, that would go against what we've seen of Sangwoo's character so far and what has happened in the last few chapters. Sangwoo just adamantly claimed he and Bum are lovers in front of many cops. Going back on that or claiming Bum hauled and burried a body on his own and without Sangwoo's knowledge/physical help, would be highly suspicious (and would only give Yang more to hang Sangwoo with).
Officer Yang knows Sangwoo is a serial killer. He also knows Sangwoo won't talk. He also outright says he's going to trick them. I think it's pretty obvious what he's doing. The only question is whether it's ALL he's doing: there could be more the trick, maybe having something to do with taking Bum's DNA, the knife, and liquid sample. Is that part of the trick, or is he just gathering evidence?
All he people in the comments who've never read or watched a cop show....The cops are tricking Bum into a confession. It's the oldest trick it the book, along with good cop-bad cop. Sangwoo didn't confess anything.