Okay, bumping this because I checked through the thread and didn't find anyone with my theory.
Alright. Well.
I mentioned in the pairings thread that I think Momorin and Date were in a relationship at some point, a long time ago.
It's relatively safe to assume they've been working together for some time, although the actual length of time is debatable. They also seem to be of approximately similar age. I imagine that they had a relationship at some point -- in my mind, probably not for very long -- and Sue's father came into the picture later on.
Although I think Date was always a bit of a jerk, I can foresee Momorin's affection toward Sue's father making him grow even more bitter and resentful.
At the end of the day, I honestly believe the reason he was specifically seeking out Sue at the start of the game was largely an act of revenge (although I can accept the theory that he wanted to test out the effects of the flower specifically on a human-turned-mimiga). What tipped me off was when Momorin mentioned that "men who don't treat women with respect are barbarians" (referring to the doctor) -- definitely made me think she's seen/put up with his bullshit before.