The first film of Park Chan-wook's vengeance trilogy, followed by Oldboy and Lady Vengeance.
Sympathy for Mr. Vengeance
(2002)

Ryu, a deaf mute factory worker, needs money so his sister can get a kidney transplant. When a suitable match is unavailable, he goes to Black market organ brokers who rip him off.

Then, a suitable match comes up and he has to raise the money quickly. So, per the suggestion of his leftist girl friend, Yeong-Mi, he kidnaps the young daughter (Yu Sun) of a CEO (Dong-jin). Ryu's sister finds out what her brother is doing and kills herself out of guilt.

While Ryu burys her by a river, Yu Sun accidentally drowns. Ryu finds the organ brokers and kills them. Dong-jin finds Yeong-Mi and tortures her to death, despite being warned about her leftist friends. Dong-jin finds Ryu and drowns him. Yeong-Mi's leftist friends find Dong-jin and kill him.

Thanks Don P!