Friend
(2001)

Dong-Su (Jang Dong-gun) is killed by members of his own gang in a power move.

Joon-suk (Yoo Oh-sung) is eventually brought to trial for the murder. Though his friends Sang-taek (Suh Tae-hwa), Joong-ho (Jung Woon-tak), and Dong-Su's father believe him to be innocent of the crime, Joon-suk pleads guilty, apparently out of remorse for not being able to prevent his friend's death.

Joon-suk is taken to prison. The last sequence is a flashback of the four friends in happier times.  

Thanks Tim (Spectre)!