Obviously, the entire 'murder' thing with Ryougi is an extended metaphor for repressed sexuality as a result of growing up in a very prudish, traditional family. In MSP1, Shiki/SHIKI manifests a tatemae/honne dichotomy so extreme as to come across as different personalities entirely. Caught in the conflict between the expectations placed on her behaviour and her growing-to-be-overwhelming attraction to Kokutou, she eventually has a psychotic break and tries to rape him:

Fortunately, 'Shiki', who embodies the values she's had beaten into her since childhood, wrests control and immediately tries to commit suicide out of shame. The entire rest of the story is an internal conflict centered around sex addiction. MEoDP are a metaphor for constant intrusive thoughts of a sexual nature (I had a joke about Mystic Eyes of 'D' Perception, but I've got more class than that, don't I?) The conflict is between the traditional values espoused by her grandfather ('you can only kill one person in your life' can here be interpreted as 'monogamy 4 lyfe, otherwise you're a useless whore') and her own desire to slut it up as much as possible. The latter manifests through her persistent attempts to lose her virginity to a string of casual, self-destructive relationships with:

A runaway street kid she just met one night...

...a much older guy who's into some kinky bondage shit and only wants her for her body...

...and some loser drug dealer she once knew from school. Eventually, however, with Kokutou's support she's able to overcome her addiction in order to be the pure waifu she knew she always had it in her to be. tl;dr - murder is sex, Ryougi is a repressed nympho, and Kara no Kyoukai is a paean to traditional Japanese gender roles. Search your feelings, you know this to be true.