In the end of the movie Sympathy for Mr. Vengeance, Park Dong-jin get's killed by the group of communists, that Yu-sun is member of. But earlier in the movie it is explained, that Yu-sun is the only member of this group. So why do they show up in the end? Was it simply a mistake of the inspector, who earlier stated they wouldn't exist?
Although it is true that Dong-jin is not part of the same organization (communists? terrorists?), it is worth remembering that Ryu's girlfriend, Yeong-mi, is.
When Dong-jin captures Yeong-mi and interrogates her to find out what happened to his daughter, Yeong-mi explains that she belongs to the same group, and that if he should harm her, the group would track him down and kill him.
True to her word, once Dong-jin has killed her through electrocution, the organization does indeed hunt him down and stab him to death.
So, the connection was not through his daughter, but through Ryu's girlfriend.