For those who are confused..

So I reread this latest chapter for like 10 times for me to understand what the hell is actually happening.

So Cain is in the middle of surgery (having intercourse) by using Gyeongjoon's soul and using a lil bit of his soul to heal Cain and remove the bullet (since Gyeongjoon's soul is powerful)
So then it should be a success but the doctor got attacked so the process was interrupted.

So instead of removing the whole bullet from Cain's heart and having him and Gyeongjoon's soul exit Cain's consciousness safely, the doctor got hurt so the surgery stopped;

So only HALF of the bullet was able to be removed, and a side effect of the interrupted surgery is that Gyeongjoon's soul was being consumed by Cain's Consiousness so he started to disappear and Cain is forcefully waking up because of the interruption.

And Cain realized that something was wrong and maybe he concluded that Gyeongjoon is dying because his soul is being consumed by his consciousness so he will never wake up. Good thing before Cain's soul took off he reached out to Gyeongjoon and resulted him being able to regain his soul's form.

That's why in the end Cain said that:
"Let's go back together, in exchange you'll have to promise me you'll open your eyes again"

So in the end their soul's came back together, Gyeongjoon is still alive but we'll never know how long he will be in a coma or will he wake up immediately? And there is still half a bullet inside Cain's heart but at least he regained maybe 80-90% of his power so he's able to patch his sliced half back to his body.

And also notice there is a new character, I'm not sure if Satan's Spy or an Angel
the one who is looking out and said:
"How cleean~, Oh it still not finished??" I think he/she is pertaining to the unfinished removal of the bullet inside Cain's heart.

There you go, hope those who're confused by the latest chapter may understand what is actually going on. ლ(´ڡ`ლ)