Sexy Losers/WMG

Shiunji was in love with Yuuko before she committed suicide, but he became the mask afterward.

In the very first strip of the Suicide Girl storyline, Shiunji specifically asks Yuuko if she wants to have sex before she jumps. It's possible that he was merely pretending to be into necrophilia as a plot to prevent her suicide. His tears after her death are genuine tears of sadness, but that's also exactly when he snaps. His necrophiliac father's influence finally catches up to him, and he has never looked back since, as evidenced by Mr. Watanabe saying he knew he didn't raise his son to be a "quitter." Now, why would he have that sort of reaction if Shiunji was already a necrophiliac? He must have been proud that his son finally accepted and expressed a desire to share his fetish, and one strip later, we see that Shiunji has only just decided to learn about taxidermy, and he later calls Dr. Lovetalk for advice, adding to the evidence. There is Shiunji's previous "girlfriend" who he'd supposedly been "dating" for a year before Yuuko, but it could have been someone who Mr. Watanabe set him up with. This would all mean that in a way, Tomoe is right; Shiunji is suffering from a mental sickness.

...okay, all this was probably just a case of characterization marching on, but it would make for an interesting development, wouldn't it?