*spoilers* - It was planned all along

The last scene tells us more than our two guys being buddies. I believe this was planned all along, a little bit like infernal affairs (which the departed ripped for the ones crying new world ripped the departed...).

Ja Seung was a thug, he had the upper hand over Jung (as you see the other one running away from the fight). Jung stayed in the mob and Ja Seung went cop just to do what he planned to : pose as an undercover to root out all the other cops.

They were more than buddies, they were partners, which is why Jung didn't kill Ja Seung and why, also, he told him to pick a side.