Subject term originally and specifically refers to the family of the victim in a murder case.  Now it may further refer to a sufferer of any accidents, not just limited to a bereaved family of a murder case. 

But in Mainland China now the term 苦主 has another meaning that is often used in sports.  It means some player or some team is always an invincible rival to certain opponents.  For example, the other day I mentioned US women vollyball team almost always won over China in recent matchups, despite the fact of China had won three consecutive titles in major tournaments of the world.  But the latter beat the former twice in a row in World championship this time,  so the headline of news may go like this: "China women volleyball team finally beat her biggest 苦主."  Is it interesting?