I already read chapter 19 and i just want to say this why almost every c...

To keep you reading lol(≧∀≦)
I don't understand how Mr killer calmly asked ahnin to carry on with his plans as he poured oil over his head

so truee-
and now im addticted and curiosity hits me hard (╯°Д °)╯

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In a way, I think this could be a very clever and manipulative way to drive Ah-in into a corner. By setting things up this way, he may be putting Ah-in in a position where, if he can't bring himself to set the killer on fire now, he will subsequently feel indirectly responsible for whatever happens / whatever Seo does afterwards.
Or he might be trying to make Ah-in feel more at ease by this gesture, by demonstratively giving him some control and choice in the situation.
Or maybe it could be a bit of both?
But either way, it seems to me like he's trying to draw Ah-in in to become his accomplice, whatever the means or reasoning behind that...

It’s a bet. If Ah-in continues to set the fire then he will be responsible to his death, i.e. he will be the killer. Mr.Killer probably thinks Ah-in won’t be able to do so, and so am I. Meanwhile, my guess is that at the same time if Ah-in give up setting the fire, Mr.Killer will think Ah-in chooses him over that bastard, and may even has more favor to him. Anyway Ah-in is the first who is clever enough to play with him so far

I guess the answer is in the title
It's a thrilling play, a game to him of some sort, at least for now