beta
I think that Taeshin pretended to be Haewon, met Wujin under these circumstances, Wujin knows Haewon somehow from childhood or high school or something, and thought he was the same person and started a related with Taeshin thinking this, but found out quickly it was a lie. When he exposed Taeshin and wanted to know where the real Haewon was, Taeshin felt like he was losing everything and killed himself.
Or, we have a corrupt prosecutor and Taeshin got too close to some information and in order to protect Haewon and Wujin, he killed himself because he was being threatened.
Just speculating.