A Love to Kill (2005)

Kang Bok Gu, a tough K-1 fighter with a tragic past, falls in love with Cha Eun Seok, a TV talent and actress. His love is forbidden because of a misunderstanding that led to his emotionally wounded brother’s accident. Fueled by hatred and revenge, Bok Gu becomes Eun Seok's private bodyguard, determined to destroy her. However, as he carries out his plan, his feelings for her grow deeper, complicating his desire for vengeance. Bok Gu, a cold and detached man, has long used martial arts as an escape from past heartbreak. (Source: MyDramaList)