Sun blames two people for Jin’s “death”. First of all she blames her father, she has removed him from power by taking over his company. Secondly I believe she blames Ben for blowing up the freighter.
As far as we know Locke and Ben are the only two people who know that Ben killed Keamy and subsequently blew up the Freighter. I believe that Locke may have visited Sun, as Jeremy Bentham, and told her this fact. After that Sun will have wanted revenge against Ben, she would be trying to hunt him down. Who better to help her than Ben’s nemesis, Charles Whidmore.
So Sun went to Whidmore asking him to help her. She said: “We both have common interests” and “we aren’t the only ones who got off the island”. I think this implies that they are going to work together or at least try to take advantage of each other to find and hurt Ben.
On a side note: Jack says that Sun blames him for Jin’s death, I think this is just Jack feeling guilty, I don’t believe Jack is the second person that Sun blames.
For more info see the Original Post
This is probably the first time that I’ve been 100% right. Unfortunately I couldn’t care less about the theory.