Question and spoiler.

Near the end of the film, Jang finally finds the pieces of papaer that had the murder orders from Mr Yoo. That was the piece of evidence he and Oh needed to convict Yoo, but he just goes to try to kill him instead of presenting the paper.

Why he didn't try to use the evidence he found?

Also, the way the whole thing about Yoo killing one witness and making the other appear as if he had ben beated was a bit disjointed. As if there was no one watching neither of them, or as if he had bought almost all the police. That part wasn't very well constructed in the script, and the whole feeling in the film (and the ending) that the only solution was going vigilante took strenght from the film.

"I believe the common character of the universe is not harmony, but hostility, chaos and murder."