You don't explicitly understand why anybody *beep* does anything in this movie. None of the characters motivations are spelled out, you're left to wonder. You have to actually think about why they do the things they do to try to understand them. Just like in real life.
I think the ambiguity of the plot is necessary when taken into the context of the ending, where we find out Sunwoo imagined the events of the film. The disciple/master story at the end pretty much states this.
You can also guess at their intentions. When Sunwoo meets the girl for the first time, you can tell he is smitten. When he is about to call the boss, the hesitates when he sees her move her hair the way she did.
This would indicate that maybe he likes her. Or perhaps he got to know her too much and got close, and didn't want to see any harm come to her. That's my best guest for Sunwoo's motivation.
As for the boss, he states in the opening scene at dinner that you can do 100 things right but one mistake can mess up everything. This basically foreshadows events of the film. He even instructs Sunwoo to kill his girlfriend and her lover if she is cheating on him. They basically set up his bad reputation early on, to show how ruthless he is.
The disciple/master story doesn't state it was all a dream at all. And in my humble opinion, the events of the film are real.
The master/disciple story clearly states that the disciple was not having a sad dream or a nightmare. It was a beautiful dream that made the disciple cry as he feels he can never live it.
So, keeping that in mind, if all of the events of the film are a beautiful dream for Sun-Woo like it was for the disciple, then it should be filled with all of Sun-Woo's hopes and happiness right? But what happens? He is betrayed, he is beaten, he is humiliated, he is buried alive, his hand is crushed, and in the end he is shot to death. What a fantastic "dream"!
No. Sun-Woo is not dreaming. The end master/disciple quote is Sun-Woo thinking what it must feel like to be free like the girl wanted to be. Just before he dies he thinks back to the point just before all this started and what it would have been like and how he would behave if his life wasn't what it was. His life has been crap and now he's almost dead, but for one moment he can clearly see (now dreaming) how happy he could have been in life but also knowing he will now never have the chance to live like that.
The director already said the movie was not a dream, not to mention the movie never implies this. The phrase at the end is just an analogy of the events in the life of SunWoo, it doesn't mean it was something that he was dreaming.
My take - He was a one-dimensional bodyguard type character and he knew it (he was even offended when he was asked if he was an enforcer).
Whether he was in love with the girl, just infatuated or just loved the idea of seeing that there was more to life than just being a servant for his boss, I think it all came down to him wishing that he was more than he was - and knowing that it could never happen.
The final 'dream-like' scene was a flashback to highlight that he was initially quite a simpleton in what he found enjoyable. He was just shadow boxing on his own as he probably imagined beating up people who he didn't think much about, no more deep than that.
Once he started caring about other peoples feelings, or thinking for himself, it all went downhill but I think he was happier in his 'bittersweet moment' that he died knowing this rather than living oblivious.
Also, I saw it mentioned by others that he was quite irrational in his revenge and quite sloppy but I think that came down to him just not thinking logically and being more emotional. So showing that to be an efficient killer he had to be quite cold.
"On a clear spring day, a disciple looked at some branches blowing in the wind, and asked, 'Master, is it the branches that are moving, or the wind?' Without even looking to where his pupil was pointing, the teacher smiled and said, 'That which moves is neither the branches nor the wind, it is your heart and mind."
It's too naive and predictable to be a "good" movie. There are no twists or anything. Within 10 minutes of watching this movie you know *exactly* where it's going. No surprises or anything, just generic. Sad excuse for excessive violence and those scenes weren't very good to begin with.
However as for the viewer not fully understanding the characters, it's kinda typical of Asian movies(or in this case Korean). I'm fine with that, but such a cheesy plot, following the rule book by the numbers. It was just too much for me. Couldn't take this movie any serious. I guess the only defense this movie has is the age, it was done in 2005. I guess I'm just too old and have seen it all.
It's mentioned early in the film that "face" is very important. Notice how guys get super insulted over minor comments in meetings. The protagonist directly disobeyed an order from the boss which deeply insulted him. So to save face among the rest of the crew he needed to make an example of him. Plus Asian culture is just different.