No pun intended - I don't know how true this is - I don't even remember this telling me anything if a true story or how much of this actually happened. Does it matter though? It is a really tight script ... it is well done ... and it has characters that you will love (to hate) ... and hate to love! They are not simple, that is for sure.
And you also will have a lot of violence ... and a lot of blood too. This gets quite nasty to say the least. You may not like what it does with a character you like .... you may not like how certain characters are treated ... or you may not care about them at all ... at least you'll see terrific stunts.
I assume the ending is supposed to hint at a sequel ... maybe that is a thing that can happen, maybe it is just to show us a possible future of a certain character. Well done - on all the levels! Technically speaking, but also from the acting point of view ... Korean filmmaking, the top tier at least or the stuff we get in the west ... is top notch!