@Bambino This is just what i've understood so far, so maybe take it with a grain of salt.
Jitae is taking down everyone involved in his mother's death, including Jian who the bodyguard protects.
The bodyguard is in love with/obsessed? with Jian but Jian's a playboy who sleeps with a ton of people. He wants Jitae to essentially destroy Jian's life and make him miserable so the bodyguard can easily swoop in and make Jian depend on him and I guess love him.